Tire Dictionary

 
by Tireman Info 16. May 2012 16:13

Air Pressure

CHECK the pressure in your tires at least monthly and before long trips when your tires are cool (after the vehicle has been stopped 3 hours and then driven less than one mile). Adjust to the vehicle manufacturers specified pressure while tires are cold. Never bleed or reduce air pressure when tires are hot. It is normal for pressure to build up as a result of driving. Use an accurate tire gauge to check pressure and maintain it at the level recommended by the vehicle manufacturer. Don't forget your standard size or temporary spare tire. Your temporary spare - it requires a higher inflation pressure. Remember: Under inflation is the most common cause of sudden air loss or sudden failures in any kind of tire and may result in unexpected loss of vehicle control or accidents.

Alignment of Vehicle

A wheel alignment adjustment may be necessary if the vehicle pulls to the right or the left when the steering wheel is in straight ahead position. Another indicator of the need for an alignment check is if tires are wearing unevenly.

All Season Tires

All season tires can be used throughout the year. The following markings appear on the sidewall of the tire: M+S, M/S or M&S. This meets the RMA definition of a mud and snow tire. However, there are also tires designed for severe snow conditions. These tires will show a symbol of a mountain with a snowflake next to the MS letters.

ABS / Anti-Lock Brake System

Under emergency braking, using conventional braking systems the wheels can lock up, making the car un-steerable. ABS systems provide continuous monitoring and control of the braking force and in some circumstances can reduce the braking distance while maintaining full car steer ability.

Modern high-quality tires are optimized and matched to the ABS functions."Braking on wet roads with ABS and ABS-brakes" are already often a standard test required by auto manufactures for many tire test specifications.

ASR / Anti-slip-control

ASR is fitted to vehicles to prevent wheels slipping, spinning on slippery or uneven surfaces.

Electronic sensors are used to control and dose the power transmitted to the drive axle, in order to ensure that tires can properly and reliably grip the road during acceleration.

Aquaplaning

The contact area of the tire to the road is reduced when by water is on the road. In extreme cases, the vehicle "hydroplanes (glides) on the water". This will drastically reduce the control of the vehicle.

Tires have special tread patterns that ensure optimum drainage of the water away from the tread surface. This effect does however reduce proportionally as speed increases.

The most effective protection is to adjust driving speeds to the weather conditions.

Balancing

At high speeds, tires generate enormous centrifugal forces. Even tiny irregularities in the tire of only a few grams are multiplied by many orders of size.

Such imbalance stresses tires and suspension. This weight irregularity can be tested and identified at tire dealerships and is balanced by adding small counter-weights.

Every time a tire is fitted to a wheel, it should be balanced.

Bead

The bead of the tire is that part which sits on the rim. At the center of the bead is the core, which comprises a bundle of steel wires embedded in rubber.

This provides a safe and solid seating of the tire on the rim.

Braking Distance

The distance required for braking depends on the speed of the vehicle, the condition of the road surface and the condition of the tires, in particular the tread. Check the tires tread depth regularly and change your tires when worn down to the "tread wear indicators" located at the bottom of the tread grooves.

Camber

The purpose of wheel camber is to reduce friction during cornering. The camber is measured when the wheels are standing on a flat surface. The difference from the vertical (inward or outward tilt of the tire) is then referred to as either positive or negative camber.

Casing

Modern tires are made of many different materials and components.

Looked at schematically, there is the outer cover - the tread and sidewall, and the substructure, the casing.

Casing components may include steel and/or textile cord plies, the inner liner (to make tube-less tires airtight), sidewalls, the apexes, the bead core (keeps the tire on the rim) and the bead reinforcement.

Chains

Even modern winter tires can sometimes not help when there are huge amounts of snow and steep gradients. In these situations traction, lateral control and reliable braking require tire chains. In order to be prepared it is recommended to try and fit chains in a "dry run".

Snow chains have to be draped over the drive wheels.

Please also note that a maximum speed is given. With some low profile tires a problem can result: the reduced space between the tires and the wheel arch leaves no room to fit snow chains.

Date of Manufacture

The date of manufacture of a tire is indicated on the tire's sidewall at the end of the DOT serial number.

Tire manufacturers have adopted a standard identification system: four numbers, which indicate the week and the year of manufacture. For example, the figures 0201 indicate that the tire was made in the second week of the year 2001.

Direction of Rotation

On standard tires with symmetrical tread patterns, it does not matter which way the tire is fitted on the rim and in which position it is fitted on the car.

Some tire manufacturers have, however, started producing tires with specific directions of rotation in order to improve wet grip and optimize noise generation.

The direction of rotation is marked on the side of the tire with an arrow. This side of the tire must be on the outside, and the tire must roll forwards in the direction of the arrow for optimum tire performance.

A number of tires with asymmetric tread patterns are also now available which do not have a specific direction of rotation.

DOT Serial Number

The "DOT" symbol certifies the tire manufacture's compliance with the U.S. Department of Transportation tire safety standards. The DOT serial number is located on the lower sidewall of the tire, on one side only. Below is a description of the serial number. Starting in the year 2000, four numbers are used for the Date of Manufacture, first two numbers identify the week and the last two numbers identify the year of manufacture. Prior to year 2000 three numbers are used for the Date of Manufacture, first two numbers identify the week and the last number identifies the year of manufacture. To identify tires manufactured in the 90's a decade symbol (a triangle on its side) is located at the end of the DOT serial number.

For Example: DOT NJ HR 2AE2 529

529

Date of Manufacturer, example: 529 (52nd week of 1999) or 5200 (52nd week of 2000).

2AE2

Tire Type Code (coding for type of tire optional by manufacture).

HR

Tire Size Code Number.

NJ

Manufactures Plant Identification Code

DOT

Reference Symbol (certifies the tire manufactures compliance with U.S. Department of Transportation tire safety standards).

ESP / Electronic Stability Program

An Electronic Stability Program, ESP, helps master critical driving situations, for example when the vehicle suddenly over steers during cornering or when sudden evasive action is required. The systems detects skidding movements within fractions of a second and can take corrective action.

ESP systems not only function when road conditions are good, but also on wet, on icy and on unpaved roads.

Technically speaking, The ESP system combines the ABS / Anti-Lock Brake Block System, electronic braking pressure distribution, ASR / Anti-slip-control and yaw control.

Emergency Mobility Systems

If a tire punctures and looses air, a standard size or a temporary special spare tire must be put on in order to continue the journey.

In order to avoid the troublesome, sometimes dangerous procedure of changing a tire on an open road, various manufacturers now offer so-called emergency mobility systems.

What these tires have in common is that when all air pressure is lost, the rim does not destroy the tire. The journey can be continued without changing the tire - over a limited distance at a restricted speed.

Load Index, Ply Rating or Load Range

These symbols are found on the sidewall of the tire indicating the load - carrying capacity of the tire.

Mixing Tires

It is recommended that all four tires be of the same size, construction and speed rating. If tires of different speed rating are mounted on a vehicle, the vehicle speed capability will be limited to the lowest speed-rated tire on the vehicle. It is recommended that the lower speed-rated tires be placed on the front axle regardless which axel is driven. This should be done to prevent a potential oversteer condition. Vehicle handling may also be affected. Consult the tire manufacture.

Radial Tires

Radial tires have body cords that run across the tire nearly perpendicular to the beads. Radial tires have belt plies, which are laid diagonally under the tread to stabilize and strengthen the tread area and add flexibility to the sidewall. By restricting tread movement during contact with the road, the belt plies increase improve tread life, traction and handling.

Reinforced or XL (extra load) Tires

Reinforced or XL (extra load) tires are specially reinforced tires. They can carry higher loads than a tire of the same size.

Reinforced tires are designated on the Sidewall by the letters "RF", extra load tires with the letters "XL"

Reinforced and XL tires require need higher inflation pressures compared to standard tires.

Revolutions Per Mile (RPM)

The number of revolutions a tire makes in one mile, at a given load, speed and inflation. Sometimes called RPK or revolutions per kilometer.

Rolling Resistance

The drag force required to put a free rolling tire into motion. Tires are not rigid, but flexible. During driving the tires compress and flex.

This flexing absorbs energy, converting it into heat.

In order to reduce rolling resistance, manufacturers use special rubber compounds. Any reduction in the rolling resistance of the tire helps reduce fuel consumption.

Since rolling resistance also increases with low inflation pressure, it is beneficial to check the pressure of tires regularly.

Rotation

Refer to your Vehicle Owners Manual for recommended rotation pattern and interval for your vehicle. If not available, follow one of the patterns shown below. It is recommended to rotate your tires every 6,000 to 8,000 miles, or sooner if uneven treadwear begins to appear. The purpose for regular rotation is to achieve more uniform treadwear on all tires on your vehicle. If tires show uneven treadwear, ask the serviceperson to check and/or correct any alignment or other mechanical problem before rotation.

This is true for both front wheel and rear wheel drive vehicles. Full size spare tires should be included in the rotation pattern for your vehicle. Compact spares (temporary use spares) should not be included in the rotation pattern.

Speed Symbol

Speed ratings for tires are identified by means of a speed symbol shown on the sidewall of a tire. The maximum speed for these symbols in shown in the table. Although a tire may be speed rated, tire manufactures do not endorse the operation of any vehicle in an unsafe or unlawful manner. Furthermore, tire speed ratings do not imply that a vehicle can be safely driven at the maximum speed for which the tire is rated, particularly under adverse road and weather conditions or if the vehicle has unusual characteristics.

Speed ratings are based on laboratory tests, which relate to performance in the road, but are not applicable if tires are underinflated, overloaded, worn out, damaged or altered.

Example: Tire size P215/60R15 H 185/65 R 15 H: the H indicates a maximum permitted speed of 130 MPH.

Temporary Spare Tires

Temporary spares are designed to carry the same load as the standard size tire on your vehicle and can be applied to any position. Maintain the proper inflation pressure as shown on the sidewall of the tire, it requires a higher inflation pressure than a standard size tire. Refer to the information on the sidewall of the tire for proper usage. With such a tire, a vehicle may be operated until it is convenient to repair or replace the disabled tire. Have your standard tire repaired or replaced as soon as possible, then return the temporary spare to the trunk to conserve its useable tread life. The temporary tire can be worn down to the tread wear indicators, same as your standard tire. At such time the tire must be replaced.

Toe

The toe describes the distance between the centerlines of the tires on an axle. The toe setting can be adjusted on all cars.

Since most wheels tend to run towards the outside because of the camber, most cars are set with a slight positive toe-in. This means that the wheels are slightly closer together at the front than at the back.

Incorrect settings for your vehicle result in uneven tire wear. If you notice uneven tire wear, then have your vehicle alignment settings checked.

Tread

The tread is that part of the tire with the groove pattern which is in contact with the road. The tread is specifically designed to provide traction for stopping, starting, cornering and provide long lasting wear.

Tread Depth

The measured distance from the tread surface to the bottom of the main grooves away from the Tread Wear Indicators. Usually specified in 1/32 of an inch.

TWI (tread wear indicator)

Tread wear indicators ("wear bars") are located at the base of the main grooves and are equally spaced around the tire. Always remove tires from service when they reach a remaining tread depth of two thirty-seconds of an inch (2/32"). If not corrected, wet weather accidents are more likely to happen due to skidding on bald or nearly bald tires. Also, excessively worn tires are more susceptible to damage from road hazards. Built-in treadwear indicators, or "wear bars," which look like narrow strips of smooth rubber across the tread, will appear on the tire when that point of wear is reached. When you see these wear bars, the tire is worn out and it's time to replace the tire.

Tire Size Designation

The dimensions of a tire are detailed on the sidewall.

In the case of a P185/65R 14 tire, the figures mean the following: 185 = width of tire in mm; 65 = the ratio of the height to the width as a percentage; R = radial construction; 14 = diameter of the rim in inches.

Tire Storage

Tires should be stored in a dry, cool place, away from sunlight and sources of ozone, such as electric motors.

If you must store tires flat, (one on top of the other), make sure you don't stack too many on top of each other. Too much weight can damage the bottom tire.

Also be sure to allow air to circulate around all sides of the tires, including underneath, to prevent moisture damage.

If storing tires outdoors, protect them with an opaque waterproof covering and elevate them from the ground. Do not store tires on or over black asphalt or other heat-absorbent or reflective surfaces, such as snow-covered ground or sand. Solvents, fuels, lubricants and chemicals should be kept out of contact with tires.

Spare tire carriers on your vehicle are not intended to be used for long term tire storage. If your vehicle has a full size tire (same size and type tire recommended for use by the vehicle manufacture not temporary use spares) as a spare, it should be included in the tire rotation pattern.

UTQG (Uniform Tire Quality Grading)

Treadwear

The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test track. A tire graded 200 would wear twice as long on the government test course under specified test conditions as one graded 100. It is wrong to link treadwear grades with your projected tire mileage. The relative performance of tires depends upon the actual conditions of their use and may vary due to driving habits, service practices, differences in road characteristics and climate.

Traction

Traction grades, from highest to lowest, are AA, A, B and C. They represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete.

Temperature

The temperature grades, from highest to lowest, are A, B and C. These represent the tire's resistance to the generation of heat when tested under controlled conditions on a specified indoor laboratory test wheel.

Valve

The valve, fitted in the wheel, ensures that the tire can be filled with air. The correct valve is required for the correct wheel/tire assembly; this is the job of the tire dealer. The cause of a slow loss of air pressure can be a defective valve. The valve cap should always be fitted to the valve in order to protect the valve core from dirt and moisture.

Valve Cap

The valve cap, although small, has a very important job: it protects the sensitive valve internals from dust, dirt and humidity. If valve caps are lost they should be replaced immediately in order to avoid expensive damage later.

Winter Tires

In snowy areas, many cities and counties have "snow emergency" regulations, which are invoked during heavy snowfalls. Check with authorities for the rules in your area. Under some rules, motorists are subject to fines if they block traffic and do not have snow tires on their vehicles.

You can avoid this by equipping your vehicle with snow tires marked with "MS," "M&S," or "M + S" on the sidewall.

If you change to snow tires, be sure they are the same size and construction type as the other tires on the vehicle.

Snow tires should be used in pairs (or as duals) on the drive axle (rear drive vehicles only) or on all four-wheel positions. Never mix non-radial snow tires with radial tires. On front-wheel-drive or performance vehicles, it may be advisable to install snow or all season tires on all wheel positions to maintain consistent handling in snowy conditions.

In areas where heavy snowfalls are frequent, many drivers carry chains for use in emergencies, or have their tire dealer apply studded snow tires or install tires for use in severe snow conditions.

Most states have time limits on the use of studs. Before installing studded tires, check the regulations in your area. If studded tires are applied to the front axle, they also must be applied to the rear axle.

If you use chains, make sure they are the proper size and type for your tires, otherwise they may damage the tire sidewall and cause tire failure.

Tires designed for use in severe snow conditions generally have tread patterns, structure and materials to give superior performance. These tires are marked with the "M+S" designation plus a mountain/snowflake symbol.

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Tire Dictionary

 

Frequently Asked Questions

 
by Admin 13. March 2012 13:45

Question: What does UTQGL mean?

Answer: UTQGL stands for Uniform Tire Quality Grade Labeling, a system that classifies tires as to treadwear, traction and heat resistance. Each tire manufacturer performs its own tests in these categories, in accordance with government prescribed test procedures. Based on these tests, each manufacturer then assigns grades, which are branded on the tire sidewall, Example: Treadwear 400 Traction AA Temperature A

 

Question: How is the tread wear grade determined?

Answer: Treadwear tests are performed on a government-prescribed 400-mile section of public highways near San Angelo, Texas. Test vehicles travel the same course at the same time, so all tires experience the same conditions. During the test, tread groove depths are measured every 800 miles. The same procedure is followed with a set of reference control tires. After 7200 miles of testing, the tread depths of test tires and reference control tires are compared and the test tires are graded on the basis of relative wear.

 

Question: If my treadwear grade is 400, how long will the tire last?

Answer: The best way to understand a treadwear grade is to compare the grade of one tire with another. For instance, a tire with a treadwear grade of 400 might be expected to last twice as long as a tire with a treadwear grade of 200. However, there is no way of accurately predicting how long your tires will last. This is determined not only by tire quality, but also by road surface quality, personal driving habits, tire inflation pressures, wheel alignment and frequency of tire rotation. The treadwear grade is only a reference point to indicate how one tire performs in relative terms to another on the government-controlled treadwear course. It was never intended to project the exact mileage a particular tire might deliver.

 

Question: The traction grade on my tire is A. what does that tell me?

Answer: Put simply, it grades the tire's ability to stop a car in a straight line on a wet test surface. For example, a tire with an AA grade will stop more quickly in a straight line on wet pavement than a tire with a C grade. Note that these traction tests are performed on government-maintained concrete and asphalt skid pads that have a specified degree of wetting to simulate most road surfaces. These test do not measure braking under dry pavement conditions, or cornering traction under any conditions. Traction grades range from AA, A, B to C with AA being the best.

 

Question: Is the temperature grade on my tire important?

Answer: Yes, it represents a properly maintained tire's ability to dissipate heat under controlled indoor test wheel conditions. A tire is graded "C" if it meets the minimum performance required by the Department of Transportation (DOT). Grades of "B" and "A" represent higher levels of performance than the minimum required by the DOT. All tires must meet the minimum speed requirement of 85 mph set by Federal Motor Vehicle Safety Standard 109.

 

Question: Is tire rotation helpful?

Answer: Yes, because tire rotation can provide more even tire wear, maximize tire life, and provide better handling for the life of your tires, if the correct tire rotation guidelines are followed. For example: if you drive a front-wheel-drive car, your front tires absorb most of the forces associated with load, acceleration, driving, steering and braking, all of which contribute to tire wear. The difference in wear rate between the front and rear tires on a front-wheel-drive car can be as much as three to one. To obtain relatively equal mileage on front and rear tires, you should rotate your tires every 3,000 to 5,000 miles. If you notice any signs of premature, irregular or uneven wear patterns, rotate the tires more often. Ask your tire dealer to recommend a tire rotation pattern that is right for your vehicle.

 

Question: May I use my "temporary spare only" tire in my tire rotation pattern?

Answer: No, temporary means just that. For proper and safe rotation, only use tires of a like construction and size. Temporary spares are of a different size and construction than the other tires on your vehicle and require much higher inflation pressures. However, if you have a regular full-size spare in your trunk, you may include that in you rotation pattern. Check with you tire dealer for proper procedures.

 

Question: Is tire-sizing nomenclature complicated, or is it just me?

Answer: It is assuredly not just you! Four sizing systems are in use for passenger tires: P-Metric, European Metric, Millimetric and Alpha Numeric. The most widely used in the U.S. is the P-Metric system. If the size branded on your sidewall is arranged as follows P215/65R15 95S (the specific numbers may vary), then you have a p-Metric size tire. The P stands for passenger car tire; the 215 is the tire's "section width" - its width at the widest point - in millimeters; the 65 is the tire's "aspect ratio" - the percentage of its sidewall height relative to its section width - and the R is for radial construction. The 15 is the rim diameter, in inches, and the 95S is the service description (95 being the load index - which corresponds to a table of maximum load capacity in pounds - and S being the speed rating). And this just describes one sizing system; we have three more to go!

 

Question: Then, what's the European or Millimetric size?

Answer: Essentially, this system is a conversion of the original (and now obsolete) numeric sizing system from inches to millimeters. If the tire size on your sidewall is arranged like 185/70R14 88S, (again, your specific numbers may vary), then you have a European Metric size. The 185 is the tire's section width, in millimeters, while the 70 indicates aspect ratio. Next, the 14 is the rim diameter, in inches, and the 88S is the service description (load index and speed rating). R simply indicates that it is a radial construction.

Your tire is a millimetric size if the size is arranged like 240/55R390. In this example the 240 is the section width in millimeters, the 55 is the aspect ratio, the R means the tire is of radial construction and the 390 is the rim diameter, in millimeters.

 

Question: What should I know about light truck tire sizing?

Answer: There are three light truck sizing systems in use today. The LT-Metric light truck sizing system mirrors the P-Metric system for passenger tires. For instance, a size such as L235/75R15/C on your sidewall breaks out this way: the LT stands for light truck; the 235, your tire's section width in millimeters; the 75, aspect ratio; the R, radial construction and 15, the rim diameter in inches. Last, the C represented the tire's load range.

Keep in mind that nearly 50% of all new vehicles built in the U.S. are not "cars" in the traditional meaning, but includes minivans, pickups and SUVs. Nonetheless, many are being fitted with P-Metric passenger car tires, which tend to emphasize ride comfort and fuel economy. However, many light truck vehicles require higher load carrying capacity than offered by P-Metric tires primarily for commercial purposes, and thus come fitted with light-truck-sized tires.

 

Question: The tire size on my pickup reads 31x10.50R15LT/C. What does this mean?

Answer: It means you have a light truck "flotation" tire, designed to deliver better traction on sand and soft soil found in watery off-road situations. The 31 indicates the tire's overall diameter in inches while the 10.50 shows the tire's section width in inches. The R means the tire is a radial, the LT stands for light truck tire and the C indicates the tire's load range.

 

Question: When is the light truck numeric sizing used?

Answer: This older sizing system is still used on older commercial vehicles. The tire size branded on the sidewall looks like this: 9.50R16.5SLT/D. In this example, the 9.50 represents the tire's section width in inches; the R, radial construction; the 16.5, the rim diameter in inches; the LT, Light Truck and the D, the tire's load range.

 

Question: What do I need to know about aspect ratio?

Answer: Simply put, the lower the aspect ratio, the shorter the sidewall, and in most cases, the quicker the steering response. In engineering terms, a tire's aspect ratio is the dimensional relationship of the tire's section height to section width, expressed as a percentage. For example: a tire with an aspect ratio of 75 has a sidewall which is 75% as tall as the tire is wide, at its widest point. A 50 aspect ratio (also called a 50-series tire) is half as tall as it is wide.

 

Question: How do I know when I need new tires?

Answer: Ask a Tireman tire expert to replace your tires if the tread depth is 2/32nds of an inch or less as indicated
by the tire "wear bars" molded into the tread grooves. Also known as treadwear indicators, tire wear bars are raised areas in the tread grooves, which become even with the tread surface when the tire is worn to 2/32nds. You can't miss them. Most states require replacement of tires worn to this tread depth because of the increased possibility of tire failure, sudden traction loss in the rain and virtually no traction in snow.

In southern states where torrential downpours test a tire's ability to get rid of water through its tread grooves, you would do well to replace tires before they reach 2/32nds. In snow belt areas, replacement before you get to the wear bars is also wise. In deep snow, the tire must be able to compress and clean out packed snow from its tread grooves.

 

Question: How should I choose tires right for me?

Answer: Start by checking the vehicle tire placard or your Vehicle Owner's Manual. Both list the original equipment and optional tire types and sizes suitable for your vehicle. However, let's say your SUV originally came with P-Metric all-season tires and you live in a heavy-snow-belt area. Think about replacing your all-season tires with a more aggressive LT-Metric type tire. Or, if you live in the farm belt where mud is an issue, think about a mud-terrain type tire. Because there are so many tire types to consider, it may be wise to discuss your tire needs with a Tireman Expert.

 

Question: Who should install my new tires?

Answer: Tireman has professionally trained tire installers at all locations. Additionally, our investment in the latest equipment available ensures we're prepared to handle lower profile tire and wheel combinations found on many of today's vehicles.

 

Question: How do I know what inflation pressure to use?

Answer: First, check the vehicle placard in your vehicle. The automobile manufacturer has already determined the best inflation pressure for use in your tires under standard operating conditions. The air pressure should never be below the minimum listed on the vehicle placard or above the maximum recommended inflation pressure branded on the tire sidewall. Our advice is to use the pressure listed on the placard.

 

Question: How often should I check inflation pressure in my tires?

Answer: You should check the inflation pressure in your tires, including the spare, at least once a month and always before extended driving. Check the pressure when your tires are cold - that is, when your vehicle has been parked for at least three hours. If necessary, add air to inflate your tires to the pressure(s) specified on the vehicle placard. Since this reading will be most accurate with cold tires, drive to the nearest source of air whenever possible.

Never 'bleed" or reduce inflation pressure when your tires are hot. When tires heat up from driving, it is normal for inflation pressures to increase above recommended cold inflation pressure levels. But if you let air out of a hot tire, it will be under-inflated when it cools down.

Also use a high-quality air pressure gauge to check your tires, don't trust your eyes. You can't tell by looking if a tire is properly inflated.

A rule of thumb, for highway use, all passenger and light truck tires should be inflated at or more than 20 psi. For any 16.5-inch rim diameter light truck tire, the minimum highway inflation pressure is 30 psi.

 

Question: What might happen if I run my tires under-inflated or over-inflated?

Answer: Under-inflation can cause extreme sidewall flexing. The result may be dangerous heat buildup that can lead to premature tire failure. Over-inflation can cause your tires to be more susceptible to impact damage. either under-inflation or over-inflation may adversely affect vehicle handling and treadwear.

 

Question: What are tire speed ratings all about?

Answer: Officially, the speed rating of a tire indicates the highest speed at which the tire can carry a specified load under specified conditions. Letters from A to Z symbolize a tire's certified speed rating, ranging from 3 mph to above 186 mph. The speed ratings most commonly in use are:

Q 100 mph V 149 mph

R 106 mph W 168 mph

S 112 mph Y 186 mph

T 118 mph Z Zr speed capability above 149 mph.

ZR_When ZR appears in the tire size designation along with a service description - such as P275/40ZR17 93W - the maximum speed rating (indicated by the "W" in "93W") indicates the tire's speed rating - in this case, 168 mph.

In this latest effort to standardize tire designations, all ratings except unlimited Z-speed rated tires incorporate the speed symbol and load index in the tire's service description. Example: P225/60R15 95H. The 95H is the tire's service description indicating a maximum load carrying capacity of 1521 lbs. and a maximum speed rating of 130 mph. While all tires are speed rated to indicate speed capabilities in excess of national speed limits, Michelin North America, Inc., does not endorse the operation of any vehicle in an unsafe or unlawful manner.

 

Question: Why are speed-rated tires so expensive?

Answer: Because speed-rated tires must meet ultra-high performance demands, they generally have been engineered to provide dramatically better handling. A V-speed-rated tire, for instance, has been built with state-of-the-art tread designs, tire profiles, carcass materials and construction, using exotic tread polymers and compounds. While Q, S, or non-speed-rated tires meet or exceed all DOT requirements, they generally do not feature the advanced and costly construction built into ultra-high performance speed-rated tires.

 

Question: What if I replace and H-rated tire with an S-rated tire?

Answer: You should replace original equipment speed-rated tires with tires of the same or higher speed rating if the speed rating of the vehicle is to be maintained. If you replace H-rated tires (capable of speeds up to 130 mph) with S-rated tires (capable of speeds up to 112 mph), you need to know that the handling of the vehicle will be different, and that now, its maximum speed capability is limited to that of the lowest speed-rated tire on the vehicle. In this example, you would have lowered the speed capability of your vehicle from 130 mph to 112 mph.

 

Question: I am buying two new tires. Where do they go, front or rear?

Answer: Always on the rear. In a cornering maneuver on wet pavement, if your front tires lose grip first, your vehicle will tend to lose control by going straight, even in a turn. This is understeer, which can be controlled by slowing down and steering in the direction of the turn. This will allow your car to come back into line.

But if the rear tires lose grip first, your vehicle could spin, which is oversteer and more difficult to control. This requires you to make quick, precise steering corrections in the opposite direction of the turn, not a natural reaction. It is easier to control understeer than oversteer.

For the record, the best choice to make when replacing tires is to buy four new tires, all the same brand, type and size.

 

Question: If I buy just one tire, what should I buy, and where should it go?

Answer: The only sound reason to buy just one tire is to replace one tire damaged by an accident or road hazard, in an otherwise good set of four. You should always buy the same size, type, brand and tread design as the tire you're replacing. An all-season tire should be replaced with an all-season tire. A Mud&Snow tire should be replaced with a Mud&Snow tire an H-speed-rated tire should be replaced with an H-speed-rated tire. In this way you will enjoy a safer, more satisfying driving experience.

 

Question: What's the difference between an all-season tire and an all-terrain type tire?

Answer: An all-season tire is designed with a long lasting, aggressive tread pattern designed to get rid of water and snow, balancing the wet and snow traction capability with dry pavement performance. It generally features lots of biting edges that enhance snow and wet traction, It can even be branded as an M&S tire (Mud and snow.)

An all-terrain type tire is a light truck tire that has been designed with an even more aggressive tread pattern. This type of tire may be driven on- or off-road in virtually any type of weather and road ondition. In rain and on mud, an all-terrain tire's open, self-cleaning tread provides excellent traction, and its rugged edges grip on rocky and uneven terrain.

 

Question: Should I be concerned about tire load carrying capacity?

Answer: If you replace original equipment tires with the same size and type replacement tires, your newest tires will be able to handle the weight of your vehicle and its maximum allowable load. However, if you switch from an LT-Metric to a P-Metric or if you are changing sizes, you should consult your tire dealer. He or she will review your tire selection against industry tire load and inflation tables to make certain you aren't installing tires incapable of supporting your vehicle and its load. A replacement tire must always meet or exceed the load carrying capacity of the original equipment tire.

 

Question: Can changing tire sizes confuse my vehicle's on-board computers?

Answer: It can if you substantially change the overall diameter of your tires. Maintaining the original, specified diameters as closely as possible ensures that your on-board computers will function properly and thereby effectively manage such systems as your anti-lock braking system, traction control, fuel management system, electronically controlled automatic transmission and electronic handling stability system. Changing tire diameters sends erroneous readings to the computers. These systems won't
fail, but they will be impacted to varying degrees. If you have any questions about this potentially troublesome issue, contact your tire dealer.

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