Maintaining The Correct Tire Pressure

via dmv.org

Inflating and maintaining proper tire pressure ensures safer, more comfortable driving and better fuel efficiency. Particularly in times of high gas prices; in inclement driving conditions such as heavy rain, snow, or ice; and in vehicles of all sizes, tire pressure can make a major difference in driving, wherever you are.

Given the importance of the task, you might think it is complicated, but checking and maintaining your tire pressure is simple, provided you have a good tire air pressure gauge and source of air, both of which are available at many gas stations.
Get a Gauge

A simple tire air pressure gauge, available at most auto parts stores, for a few dollars, is adequate for the job. You do not necessarily need a digital air pressure gauge. If it is worth the $14 to $15 to you, a digital pressure gauge is easy to read and accurate. However, you should consider whether it will require batteries, and whether this would prevent you from using it.

Again, a standard pressure gauge that measures pounds per square inch (PSI) and fits easily in the glove box of your vehicle, is sufficient. Do avoid ultra-cheap models that may not give a proper reading.

As for a source of air, many gas and service stations have air available for 50 cents or so. Some of these air machines have gauges on them, and if you have no other means of measuring the pressure of your tires, these will work. However, they are typically beat and inaccurate, so have your own gauge to ensure the proper PSI for your tires and vehicle.
Checking Pressure

PSI is measured by the notches on a tire air pressure gauge or with a number reading on digital gauges. To find out what PSI is right for your tires, look on the tires themselves. In the raised writing on the side of the tires, you should find "recommended PSI" or "PSI recommended at ..." or similar, with the proper figure for your tires. Many times this number on the tire is stated as a maximum; you can also consult your owner's manual or the sticker on the door of the driver's side. When buying new tires, or getting a rotation, it's a good idea to ask what the ideal pressure is.

Recommendations may vary, but you should never inflate the tires five PSI more or less than what is recommended on the tire. Under-inflating wears out the sides of the tire, and is actually a driving hazard. Over-inflated tires will wear more quickly, and are also dangerous because of the increased possibility of a blowout. If you are unsure about the PSI for your tires, or it is unclear or worn away on the side of your tires, ask your mechanic or someone who knows about vehicles what PSI you should have for your tires.

Smaller compact and mid-size sedans typically have PSI levels between 30 and 40 PSI. Larger vehicles with larger tires, including bigger sedans, usually have higher pressure, around 45 PSI. These are general PSI figures, and the most accurate PSI for your tires is the number listed on the side. Tires should all be inflated to the same PSI for safety, proper vehicle function, comfort, and fuel efficiency.

Also, check your vehicle's tire pressure when the tires are cold. This means the tires should not have been driven on for at least three hours. If you need to drive to get air, try to drive less than a mile. Or, slightly under-inflate the tires to compensate for the warmer air inside them, and then check the pressure again when you can get a cold reading.

To get a PSI reading on your tire, place the air pressure gauge onto the tire's valve stem, the pencil-width air nozzle on the side of the tire. Try to place the gauge evenly onto the valve stem. This will allow air to escape, but once you firmly press the gauge down on the valve stem, it will stop the flow of air and give your gauge a reading, either by blowing out the metered stick with a traditional gauge, or a reading with a digital model gauge.
Adjusting Tire PSI

So you have a tire pressure gauge, and a source of air. It is best if you can park your car centered on the source of the air, which usually has a hose to reach the vehicle's tires. You may need to move the car to reach all of the tires, depending on the situation. Before you pay any money for air or start pumping up your tires, remove the caps on all the tire valve stems.

Next, you should check the pressure of all four tires, noting which ones need the most air. This will help you maintain uniform pressure in the tires, some of which may need less air. Hot weather, extreme temperatures and other conditions can cause the air in your tires to expand, and PSI can subsequently increase.

Once you know which tires need more air, you can deposit coins into the air machine, or get your air hose ready. Choose the first tire to fill, and fit the air hose nozzle onto the tire stem. When you start to place the air hose onto the tire stem, it will hit a pin inside the stem and start leaking air. You know when you have the air hose nozzle properly applied when the leaking air stops. It takes some force to get the hose locked on, but once it is in place, you will be ready to increase the tire pressure.

Some air hoses are automatic, and will release air in your tire once you have it on the tire's valve stem. Other air hoses have handles and require you to squeeze them to activate the air.

It is important to have your gauge as you fill the tire, taking the hose off somewhat frequently to check the pressure. It is extremely important not to over-inflate your tires. You can avoid this by using small bursts of air between your checks. As you increase the PSI and keep checking it, you will get a feel for how much air you are putting into the tire, and how much more you need. Once you get close to your recommended PSI, use less air, and keep going until you are at the right level.

Once you have the tires properly inflated, replace the stem caps by screwing them back on. Do not over-screw them, as they will break on the top. Tire stem caps are important to keep your tire valve stems clean and undamaged.

Tire pressure should be checked weekly, or every other week at least. Particularly with severe weather and temperature swings, tire pressure on the nicest tires with the nicest cars can still fluctuate, and must be monitored and maintained regularly for safe and fuel-efficient driving.

How to Check Air Pressure in Tires: 6 steps (with pictures)

via wikihow.com

 

How To Read Your Tire

The sidewall of your tires is filled with important information that tells you everything you need to know about your tire. The numbers can be a bit overwhelming to the untrained eye, so the best way to understand tire markings is to take an example and break it down, bit by bit.

For our example, these numbers are:
P215/65R 15 95H

(P) Service Description
The service description may not always appear on the tire, but it is important to know how it can affect your vehicle. If there is a "P" on the sidewall, it stands for "passenger car." This refers to the U.S. (P-metric) method of tire sizing. "LT" stands for Light Truck. "ST" is for "Special Trailer." And "T" stands for "Temporary," which is primarily used for small spare tires. If a tire does not have a "P" or another letter in front of the numbers, it is considered a "Euro-metric" tire. A Euro Metric tire conforms to the European tire specifications, and often carries a different load index than a comparably sized P-metric tire.

(215) Tire Width
The first number in this series refers to the tire's section width, or distance from sidewall edge to sidewall edge (in millimeters) when measured up and over the tire's tread. Generally speaking, the larger this number is, the wider the tire will be.

(65) Aspect Ratio
This number is the tire's aspect ratio, or its section height compared to its section width. In this example, the section (or sidewall) height is 65 percent of the section width. This number can be very indicative of a tire's purpose. Lower numbers, like 55 or less, mean a short sidewall for improved steering response and better overall handling.

(R) Internal Construction
The "R" refers to radial construction, which has been the industry standard in passenger-car tires for more than 20 years. Prior to radial tires, most cars came with bias-ply tires, which had a crude construction that made for poor handling. Bias-ply tires (which use a "B" for their description) are still used for certain truck applications.

(15) Rim Diameter
This is the rim — or wheel — diameter, in inches, for which the tire was sized. Pay particular attention to this number if you plan on upgrading your wheel size. If your wheel diameter changes, you'll have to purchase a new set of tires that matches this new diameter.

(95) Load Index
A tire's load index is a measurement of how much weight each tire is designed to support. The larger the number, the higher the load capacity. This is one of the most important numbers on your tire. To find out what "95" means, it must be looked up on a Load-Carrying Capacity Per Tire chart. In this case, 95 indicates a maximum weight of 1,521 pounds. Remember that this is per tire, which means you have to multiply by four to get the total capacity for a complete set of tires. If the vehicle has its original tires, you can just refer to the doorjamb, which lists the maximum cargo capacity with passengers.

Some vehicles are equipped with "XL" tires. No, it doesn't mean that they're extra large, but it does mean that they are extra-load tires. The load index on these tires is much higher than a standard-load tire — which is why it is important to replace an XL tire with another XL tire.

Remember "P-metric" and "Euro-metric sizing"? Their difference in load rating can lead to confusion and potential trouble. For a given size, P-metric tires will have a load index that is one or two points lower than corresponding Euro-metric tires. So if your car came with Euro-metric tires, don't replace them with P-metric tires. You can, however, replace P-metric tires with equivalent-size Euro-metric ones because you gain load capacity that way.

Why is this important? Generally speaking, you don't want your replacement tires to have a lower load index number than the originals (as indicated by the driver's doorjamb or the owner's manual), particularly with high-capacity vehicles that ride on smallish tires, such as minivans.

Also, and contrary to popular perception, optional large-diameter wheels with lower-profile tires tend to have less load-carrying capacity because they contain less air. And it is the volume of air inside the tire, not the rubber itself or the wheel material that shoulders the load. The load index is especially important when shopping for a tire online, since many retailers do not specify whether a tire is P-metric or not.

(H) Speed Rating
The speed rating is a measurement of the speed at which the tire is designed to run for extended periods. An "H" speed rating signifies that this tire can be run safely at speeds of up to 130 mph for extended periods. Will it explode if it goes to 140? No, not immediately. But it might if it is run at that speed for an extended time.

Here is a complete list of the various tire speed ratings, and their associated letters:

S 112 mph
T 118 mph
U 124 mph
H 130 mph
V 149 mph
*Z Over 149 mph
*W 168 mph
*Y 186 mph
*(Y) Over 186 mph
*The "Z" rating used to be the highest rating for tires having a maximum speed capability greater than 149 mph, but as tire technology improved, it is was ultimately split into the "W" and "Y" rating. A "ZR" may sometimes appear in the size designation, as a sort of nod to the prior rating, but it will also be used in conjunction with a W or a Y.

Additional Information on Your Tires

DOT Code
The DOT code is used by the Department of Transportation (DOT) to track tire production for recall purposes. If a tire proves to be defective, this number helps keep track of where these tires ended up so that buyers can be notified of the problem. At the end of the DOT code you'll find a four-digit number. This is the manufacturing date of the tire. The first two digits stand for the week; the other two are the year. For example, if your tire had "1610" listed, it was manufactured on the 16th week of 2010.

If you come across a three-digit number, you have a tire that was manufactured before 2000. A DOT tire code of "127" indicates the tire was made on the 12th week of the seventh year of the decade. But it's difficult to know whether that was 1997 or even 1987. According to tirerack.com, some tires produced in the 1990s may have a small triangle following the DOT number to identify the decade. But any tire that has a three-digit code is history. Tire experts recommend that tires that are six or more years old be replaced, regardless of tread depth.

Sometimes the DOT number will be located on the inside of the tire. In this case, you can either jack up the car to inspect it, or check with your local mechanic or tire shop. You should also make a habit of checking the manufacturing date on your spare tire as well.

Maximum Air Pressure
This number refers to the maximum amount of air you can put in a tire before you harm it. It is not the recommended tire pressure; that number can be found in your owner's manual and on the doorjamb.

Traction Rating
A traction rating can also be found on the sidewall of all modern tires. It can be represented as AA, A, B or C. This is a rating of a tire's traction when tested for straight-line braking on a wet surface. For this rating, AA signifies the best traction performance and C indicates the worst.

Temperature Rating
The temperature rating refers to the ability of the tire to withstand heat under high speeds. The ratings, from best to worst, are: A, B and C.

Treadwear Rating
Finally, you might find the word "TREADWEAR" on the sidewall followed by a number like 120 or 180. This is a rating of the tread's durability, as tested against an industry standard. The reference number is 100, so a tire with a treadwear rating of 200 has an 80 percent longer predicted tread life, while a rating of 80 means a predicted tread life only 80 percent as long as the industry standard.

via edmunds.com

 

Reading A Tire Sidewall

1. Read The Tire Specs

• Reading A Tire Sidewall

Maximum Load

A tire's maximum load is the most weight the tire is designed to carry. Since a tire's load carrying capacity is related to the tire's size and how much inflation pressure is actually used, maximum loads are rated with the tire inflated to an industry assigned inflation pressure.

Additionally, load ranges are used to separate tires that share the same physical size, but differ in strength due to their internal construction. 'Higher' load ranges are used to identify tires that have a stronger internal construction, and therefore can hold more air pressure and carry more weight.

Each load range has a assigned air pressure identified in pounds per square inch (psi) at which the tire's maximum load is rated. Listed below are the air pressures at which maximum load is rated for popular P-metric and LT tires:

Tire Load Ranges		Inflation Pressure Assigned For 'Maximum Load' Ratings
P-metric
Standard Load	(SL)	35 psi
Extra Load	(XL)	41 psi
Light Truck
Load Range C	(LRC)	50 psi
Load Range D	(LRD)	65 psi
Load Range E	(LRE)	80 psi

P-metric tires used on passenger cars and station wagons are rated to carry 100% of the load indicated on the tire's sidewall (or listed for the tire in industry load/inflation charts). However, if the same P-metric tires are used on light trucks, (pickup trucks and sport utility vehicles for example), their carrying capacity is reduced to 91% of the load indicated on the tire's sidewall. This reduction in load results in causing light truck vehicle manufacturers to select proportionately larger P-metric sized tires for their vehicles to help offset the forces and loads resulting from a light truck's higher center of gravity and increased possibility of being occasionally 'overloaded.'

For example, P235/75R15 P-metric sized, standard load tires used on cars and light trucks would be rated to carry the following maximum loads at 35 psi:

Cars	Full Value	2028 lbs.
Light Trucks	9% Reduced Value	1845 lbs.

Additionally, while a tire's maximum load is the most weight the tire is designed to carry, its load carrying capacity at lower inflation pressures is proportional to how much inflation pressure is used. For example, P235/75R15 P-metric sized, standard load (SL) and extra load (XL) tires used on cars would be rated to carry the following loads at the inflation pressures indicated:

Air Pressure (psi)	20	23	26	29	32	35	38	41
P235/75R15 SL	1543	1635	1753	1852	1940	2028
P235/75R15 XL	1543	1635	1753	1852	1940	2028	2105	2183

Note: 35 psi is the assigned 'maximum load' pressure for standard load tires and 41 psi is the assigned 'maximum load' pressure for extra load tires.

The above chart correctly shows that an extra load tire is not rated to carry any more load than a standard load tire when both are inflated to the same pressure (up to the standard load tire's 'maximum load' pressure of 35 psi). This is because a tire's load capacity is a function of its size (which determines the size of the 'air chamber'), its construction (which determines how much pressure can be held) and the actual air pressure used (which determines how many air molecules are forced inside the chamber). All tires with equivalent physical dimensions carry equivalent loads (until they reach their maximum load pressure).

The tire's maximum load is indicated in relatively small sized print branded near the tire's bead (adjacent to the wheel) indicating the appropriate value. Because tires are global products, their maximum load capacity is branded on the tire in kilograms (kg) and pounds (lb.). These values can also be found in the industry's tire load & inflation charts.

NOTE: P-metric and Euro-metric sized tires' 'maximum load' inflation pressure may be, and often are, different that the tire's 'maximum inflation pressure'.

Maximum Inflation Pressure

A tire's maximum inflation pressure is the highest 'cold' inflation pressure that the tire is designed to contain. However the tire's maximum inflation pressure should only be used when called for on the vehicle's tire placard or in the vehicle's owners manual. It is also important to remember that the vehicle's recommended tire inflation pressure is always to be measured and set when the tire is 'cold'. Cold conditions are defined as early in the morning before the day's ambient temperature, sun's radiant heat or the heat generated while driving have caused the tire pressure to temporarily increase.

For the reasons indicated above, It is also normal to experience 'hot' tire pressures that are up to 5 to 6 psi above the tire's recommended 'cold' pressure during the day if the vehicle is parked in the sun or has been extensively driven. Therefore, if the vehicle's recommended 'cold' inflation pressures correspond with the tire's maximum inflation pressure, it will often appear that too much tire pressure is present. However, this extra 'hot' tire pressure is temporary and should NOT be bled off to return the tire pressure to within the maximum inflation pressure value branded on the tire. If the 'cold' tire pressure was correctly set initially, the temporary 'hot' tire pressure will have returned to the tire's maximum inflation pressure when next measured in 'cold' conditions.

A tire's 'maximum inflation pressure' may be different that the assigned tire pressure used to rate the tire's 'maximum load'. For example, while a P-metric sized standard load tire's maximum load is rated at 35 psi, many P-metric sized standard load performance and touring tires are designed to contain up to 44 psi (and are branded on their sidewalls accordingly). This additional range of inflation pressure (in this case, between 36 and 44 psi) has been provided to accommodate any unique handling, high speed and/or rolling resistance requirements determined by the tire and vehicle manufacturers. These unique tire pressures will be identified on the vehicle placard in the vehicle's owner's manual.

The tire's maximum inflation pressure is indicated in relatively small-sized print branded near the tire's bead (adjacent to the wheel) indicating the appropriate value. Because tires are global products, their maximum inflation pressure is branded on the tire in kilopascals (kPa) and pounds per square inch (psi). These values can also be found in the industry's tire load & inflation charts.

Tread Depth

Tread depth is a vertical measurement between the top of the tread rubber to the bottom of the tire's deepest grooves. In the United States, tread depth is measured in 32nds of an inch. Because it is difficult to accurately measure tread depth with a ruler, tread depth is best measured with a tire tread depth gauge. When tires have neared the end of their life, a U.S. Lincoln penny can also be used to confirm the tire's tread depth. If Lincoln's entire head is visible, the tire is worn to approximately 2/32' and is considered legally worn out in most States.

Average new tires used on cars typically start with 10/32' to 11/32' of original tread depth. Dedicated winter / snow tires and light truck tires typically are deeper (for light truck tires, how much deeper depends on the tire's tread type…Highway Rib, Highway All Season, Off Road All Terrain or Off Road Maximum Traction).

As mentioned above, tires are legally 'worn out' In most States when they reach 2/32' of remaining tread depth. For example, a typical tire that starts with 10/32' of original tread depth has only 8/32' of useable tread depth. Its useable tread depth is calculated by subtracting a worn out tire's 2/32' from the new tire's original depth of 10/32'. The final 2/32' of a tire's tread depth isn't part of the equation when it comes to calculating tread depth percentages because the tire is already legally worn out with just 2/32' of remaining tread depth.

Useable tread depth is calculated by subtracting 2/32' from the tires new tread depth. Then usable tread depth is compared to remaining tread depth in order to calculate tread wear percentages. For example, a tire that started with 10/32' of original tread depth and has worn off 4/32' (down to 6/32' of remaining tread depth) is 50% worn

10/32' original tread depth
-2/32' legally worn out tread depth
8/32' of useable tread depth.

8/32' useable tread depth
-4/32' measured tread depth worn away
4/32' of remaining tread depth (50% of 8/32')

A tire that starts with 10/32' of original tread depth has 12.5% wear for every 1/32' that is worn away, and a tire that starts with 12/32' ' of original tread depth, has 10% wear for every 1/32' that is worn away, etc.

Rim Width Range

Because tires have flexible sidewalls, a single tire size will fit on a variety of rim widths. A tire's rim width range identifies the narrowest to the widest rim widths that the tire is designed to fit. The width of the rim will influence the width of the tire. A tire mounted on a narrow rim would be 'narrower' than if the same size tire was mounted on a wide rim. NOTE: Because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.

The industry rule of thumb is that for every 1/2' change in rim width, the tire's section width will correspondingly change by approximately 2/10'.

For example: a tire in the P205/60R15 size is measured on a 6.0' wide wheel and this size tire has an approved rim width range from 5.5' to 7.5' wide. The tire has a section width of 8.23' (209mm) when mounted on a 6.0' wide wheel. If that tire were mounted on all of the rims within its range, the tire's approximate section width would change as follows:

Difference from Measuring Rim	Rim Width	Approximate Tire section width
0.5' narrower	5.5'	8.03'
Measuring Rim	6.0'	8.23'
0.5' wider	6.5'	8.43'
1.0' wider	7.0'	8.63'
1.5' wider	7.5'	8.83'

Because of the different wheel widths used in the above example, there is a 8/10' projected difference in tire section width when comparing a tire mounted on the narrowest rim to the widest rim within its range. This may affect fenderwell and frame clearances when selecting optional aftermarket wheel and tire packages.

Additionally, some vehicle manufacturers and tire companies have permitted rim widths that are not within the tire's original approved rim width range. For example: BMW has combined 235/40R17 sized tires on 17x7.5' rims (which are 0.5' less than the narrowest 8.0' wide rim listed for the size) on certain M3 models; and Chevrolet has combined P255/50R16 sized tires on optional 16x9.5' rims (which are 0.5' wider than the 9.0' wide rim now listed for the size) on certain Corvette models. While these applications have received the approval of the vehicle and tire manufacturers, staying within the approved rim width range helps assure that the tire's internal stresses are within its design parameters.

Measuring Rim Width

The measuring rim width is the industry standardized rim width upon which the tire must be mounted in order to confirm it meets its dimensional targets. Because the width of the rim will influence the width of the tire, a standard rim width for every tire size is assigned and must be used. This standardized measuring rim width allows all of the tires produced around the world to meet the same dimensional standards and therefore, be equivalent with regards to their physical size. The measuring rim width is sometimes referred to as the tire's 'design rim width'.

The assigned measuring rim width changes with the tire size's section width and with the tire size's aspect ratio. As tire section width increases, the measuring rim width increases proportionately in 1/2' increments. Therefore, relatively narrow wheel widths are assigned for smaller tires while wider wheel widths are assigned for larger tires.

Tire Size	Measuring Rim Width
P175/75R14	5.0'
P195/75R15	5.5'
P215/75R15	6.0'
P235/75R15	6.5'

Additionally, relatively 'narrow' measuring wheel widths are assigned for taller profile tires (75-series sizes) which graduate in 1/2' increments to the wider wheel widths assigned for lower profile tires (40-series sizes)

Tire Size	Measuring Rim Width
P225/75R15	6.0'
P225/70R15	6.5'
P225/65R16	6.5'
P225/60R15	6.5'
P225/55R15	7.0'
P225/50R15	7.0'
P225/45R17	7.5'
P225/40R18	8.0'

Section Width

A tire's section width (also called 'cross section width') is the measurement of the tire's width from its inner sidewall to its outer sidewall (excluding any protective ribs, decorations or raised letters) at the widest point. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours.

Because a tire's section width is influenced by the width of the rim upon which the tire is mounted, the correct industry assigned measuring rim width for the tire size being measured must be used.

The width of a tire mounted on a narrow rim would be 'narrower' than if the same tire was mounted on a wide rim. NOTE: because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.

The industry rule of thumb is that for every 1/2' change in rim width, the tire's section width will correspondingly change by approximately 2/10'.

For example: a tire in the P205/60R15 size is measured on a 6.0' wide wheel and this size tire has an approved rim width range from 5.5' to 7.5' wide. The tire has a section width of 8.23' (209mm) when mounted on a 6.0' wide wheel. If that tire were mounted on all of the rims within its approved range, the tire's approximate section width would change as follows:

Difference from Measuring Rim	Rim Width	Approximate Tire section width
0.5' narrower	5.5'	8.03'
Measuring Rim	6.0'	8.23'
0.5' wider	6.5'	8.43'
1.0' wider	7.0'	8.63'
1.5' wider	7.5'	8.93'

Because of the different wheel widths used in the above example, there is a 9/10' projected difference in tire section width when comparing a tire mounted on the narrowest rim to the widest rim within its range. This may affect fenderwell and frame clearances when selecting optional aftermarket wheel and tire packages

Tread Width

The tread width is the distance between the outer edge and the inner edge of the tread of a new tire. However today's radial tires often feature tread designs that incorporate rounded shoulders and there is no industry standard pertaining to 'how much' of the rounded shoulders should be included in the tread width measurement. Because of this, it is difficult to accurately compare the tread width differences of one tire brand to another. Tread width measurements are best used when comparing the various tire sizes or lines manufactured by a single tire manufacturer. Several tire manufacturers have chosen not to publish tread width dimensions.

Overall Diameter

A tire's overall diameter is the outer diameter of the tire measured in the center of the tread. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours.

Revolutions Per Mile

Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire's published overall diameter (which would only reflect the tire's unloaded radius), calculating the tire's absolute rolling circumference isn't possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate.

 

2. Department of Transportation Safety Code

 

This assures that your tire complies with all Department of Transportation (DOT) safety standards. After the DOT insignia is your tire’s identification number, which begins with the tire’s manufacturer and plant code where the tire was manufactured (two numbers or letters). The ninth and tenth characters tell the week the tire was manufactured. The final number(s) signifies the year the tire was manufactured. 

 

  

3. UTQG Code

 

The Uniform Tire Quality Grading (UTQG) was established by the National Highway Traffic Safety Administration (NHTSA) to test tires following government prescribed test methods and then grade each tire on three main components:

Treadwear: This is the wear rate of the tire, comparable only to other tires within a tire manufacturer’s line. 100 is the baseline grade. Therefore a tire with 200 would theoretically last twice as long on the government’s course compared to a tire with 100.

 

Traction: Traction grades are AA, A, B and C (with AA being the highest grade). They represent the tire’s ability to stop straight on wet pavement as measure on a specified government track. Any tire rated under C is considered unacceptable for road travel.

 

Temperature: The temperature grades, from highest to lowest, are A, B and C. These represent the tire’s ability to dissipate heat under controlled indoor test conditions. Any tire rated below C is considered unacceptable.

 

 

4. Icons

 

Some tires have unique benefits, as showcased with specific icons. For example, the MICHELIN® Green X® Marking is a guarantee that the tire provides a level of energy efficiency among the highest in the market for its category without compromising traction and treadwear. The letters M and S (M +S) indicate that the tire meets the Rubber Manufacturers Association’s standards for a mud and snow tire. The letters can be found in the following combinations: M+S, M/S, and M&S. All-season tires carry this mark. 

via michelinman.com

via http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=205