Although many parts stores don’t sell tires, we certainly get asked a lot of questions about them. Tire-wear patterns are a great way to diagnose potential problems and sell chassis parts. Many customers come to us with TPMS-related questions, including reset procedures and compatibility. For those of us who sell custom wheels, there are questions about backspacing and bolt patterns, and there are the perennial questions like, “How big a tire can I fit under my truck without a lift kit?” and, “If I put X-size tire on a car with Y size, will it mess up my ABS?”

While some of these questions are better passed to other experts, a basic understanding of tire size and sidewall markings can be helpful in assisting customers, as well as making you a more “well-rounded” parts specialist.

As an example, let’s say the tire has these markings on it:

• P225/65/R17 
• 104S    
• M+S     
• 340BB    
• DOTxxxxxxxx

Tire Size

Aside from the tire’s brand and model name, the most obvious feature of a tire sidewall is the size. Tire size is expressed by three measurements: section width, aspect ratio and rim diameter.

Section width is the measurement of the overall width of the tire, in millimeters, from inner sidewall to outer sidewall, when mounted on an appropriate wheel. It also is known as “cross-section,” and is a three-digit number.

Aspect ratio is the “height” of the sidewall, but it’s not a direct measurement like section width. Aspect ratio is expressed as a percentage of the section width, and it’s a two-digit number. In the tire example above, the section width is 225 millimeters, and the aspect ratio of 65 indicates the sidewall height is 65 percent of the tire width. This also is known as the tire’s “profile.” The lower the aspect ratio number, the shorter the sidewall.

Although section width is a metric measurement, rim size still is expressed in inches. This most often is the only portion of the tire size we need to catalog brake or suspension components.

Of course, this tire size also includes a variety of letters, signifying the tire’s intended application and construction. The “P” prefix is the most commonly referenced service type, meaning the tire is intended for passenger vehicles, including cars, minivans, SUVs and half-ton and smaller pickups. “LT” prefixes for light trucks (usually ¾-ton and larger), “ST” for special trailers and “T” for temporary also are commonly found on the sidewall.

ST-series tires never should be used on a vehicle, as they are designed only for use on a trailer, and P-metric tires usually don’t have sufficient load capacity to withstand the heavy loading and trailer towing common to ¾-ton and 1-ton trucks. An LT may increase the capability of an SUV or truck, but unloaded ride quality may suffer due to the stiffer construction of many of these tires.

The “R” indicates the tire is a “radial” design, which will be the designation found on nearly all modern tires, with the exception of “Z”-rated and run-flat designs, which will appear as “ZR,” “RF” or “ZRF.” The “Z” rating is the only speed rating that appears as part of the size designation. For all other speed ratings, this is found in the service description along with the load index.

Load Index

In our example, the service description is “104S,” indicating a load index of 104, and a speed rating of “S.” The load index is a number from 70 to 126, giving an indication of its ability to bear a specific amount of weight. Our 104 index means this tire is capable of supporting a 1,984-pound load, as compared to a 70 (739 pounds) or 126 (3,748 pounds). The speed ratings range from “L” (75 mph) through “Z” (149 mph or greater). The “greater” the letter designation, the higher the speed rating. Our “S” rating is equivalent to 112 mph. These ratings are an indication of the tire’s performance capability, not an invitation to exceed posted speed limits!

M+S

Many tires include the “M+S” designation for mud and snow, but the “mountain snowflake” symbol is the best bet for severe snow conditions, and actually meets a specific testing standard, while M+S designations simply indicate the tread design is useful in these conditions. Both designations can be found on some “all-season” tires.

In our tire example, 340BB is the National Highway Traffic Safety Administration’s Uniform Tire Quality Grading (UTQG) rating, which encodes the treadwear, traction and temperature ratings of the tire. Treadwear is a relative score, with the tire being compared to a “test tire,” and can be thought of as a percentage. A 100 score would mean the tire tread lasts as long as the test tire, a 200 would last twice as long, etc. Our tire’s rating of 340 means the tire wear is 3.4 times longer than the test tire. Next comes traction, graded as an “AA,” “A,” “B” or “C” in straight-line wet-braking tests. Our middle “B” indicates middle-of-the road performance. The final letter (“B” in our example) represents temperature resistance. This means the tire can dissipate heat and withstand the effects of heat buildup without failing at higher speeds. This also is an “A,” “B” or “C” grade. A minimum speed of 85 mph must be achieved to earn a ”C”; our “B” tire achieves a minimum speed of 100 mph, and an “A” grade is given for tires that attain speeds of more than 115 mph without failure.

DOT Number

The final number is the “DOT” number, which identifies the tire manufacturer and the production-batch number. The number can be from 11 to 17 characters, always beginning with the letters “DOT.” These numbers are required by the Department of Transportation to identify tires in case of recall, and to certify they meet federal regulations. Tires also include information on the maximum load and maximum inflation pressure of the tire, but these maximums may exceed the vehicle manufacturer’s recommendations for vehicle loading and tire pressure. Just because a tire can handle the load or pressure doesn’t mean the vehicle can!