How to Decode Tire Size and Other Data

Tire codes hold the keys to important information

Last updated: August 23, 2015 03:00 PM

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Choosing the right-sized tire for your car is as simple as checking the tires already installed. You'll also find the recommended tire size as well as the speed and load ratings for your vehicle on a placard in the driver’s doorjamb, the glove compartment, or on the fuel-filler door. Check the vehicle owner’s manual for additional information, too.

In replacing the tires on a car bought used, it is especially important to check for the recommended size, as it is possible that the previous owner may not have installed the proper tires. For sports cars, be sure to compare front and rear tires, as they may be different.

Here's what those cryptic sidewall codes mean for a typical tire:

Size. P235/70R16 is a common one. P if displayed denotes passenger-car tire. Some may start with an LT prefix, used on heavy-duty trucks. The number 235 is the cross-section width in millimeters, while 70 is the ratio of sidewall height to cross-section width (70 percent). R means radial-ply construction and 16 is the wheel diameter, in inches.

Load index. This number is based on the weight the tire can safely carry. You'll find it after the tire size; the 104 load index for most of the tires tested for this report correlates to 1,984 pounds. Choose tires with a load index at least as high as the one that's listed on your vehicle's placard or owner's manual.

Speed rating. This letter denotes the maximum sustainable speed and is found directly after the load index. For S-speed-rated tires, it's 112 mph; for T, 118 mph. Speed ratings for other tires include Q, 99 mph; H, 130 mph; V, 149 mph; W, 168 mph; Y, 186 mph; and ZR, more than 149 mph. While such speeds may seem wildly impractical, tires with higher speed ratings tend to provide better handling at legal speed limits. Choose tires that have a speed rating at least as high as the one specified on your vehicle's placard.

Tread-wear rating. Grades for our light-truck tires ranged from 180 to over 800. In theory, a tire graded 400 should last twice as long as one graded 200. But the tire makers certify the tires meet the wear ratings.

Traction and temperature scores. These scores denote a tire's wet-stopping ability and temperature resistance. For traction, AA is best, C is worst. For temperature resistance, scores range from A (best) to C.

Maximum pressure. This is a tire's maximum safe air pressure, given in pounds per square inch. But that doesn't mean you should inflate your tires to that pressure, since automakers typically recommend an inflation pressure well below the tire's maximum air pressure. Follow the advice on the vehicle's placard.

When the tire was made. Every tire has a Department of Transportation (DOT) number following the letters on the sidewall. The last four digits determine the week and year the tire was made; for example, the digits 2214 would signify that the tire was made during the 22nd week of 2014. Don't buy tires more than a couple of years old.

See our complete tire buying guide and ratings.

Gene Petersen

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