Q. I’m getting ready to buy my first set of tires and am confused about what all the numbers mean – P-205-75-R-15, 92S, ABC, 400 – and the others. I want to draw the best comparisons between brands and stores. Any suggestions of where or how to research tire choices?

Amy Stewart,

San Jose, Calif.

A. Let’s take a look at all the numbers and letters you might find on the side of a tire, using the example you supplied. P indicates the tire is intended for use on a passenger vehicle. (LT is light truck, ST is special trailer service and T is temporary.) 205 is the section width of the tire in millimeters, and 75 is the aspect ratio (height/width). The R means a radial tire, while 15 is the wheel diameter in inches.

Current tires list the load index (capacity) and speed rating after the above group of digits (for example, 92S). The number indicates the maximum weight the tire can safely carry when properly inflated and the letter is the speed rating. The load scale ranges from a low of 60 (551 pounds) to a high of 114 (2,601 pounds). In the past, the speed rating was listed prior to the R and the load rating was listed elsewhere on the tire along with the maximum inflation pressure. The speed rating scale ranges from the letter M (81 mph) to ZR (149 mph), with the letter H (130 mph) – for whatever reason – found between U and V.

You also might see the letters M and S (M/S, M+S, MS), telling you the tire’s tread is designed for all season (snow) use. All-season tires tend to be somewhat noisier than conventional tires, and sometimes are prone to uneven wear.

Tread wear, traction and temperature each receive a UTQG (uniform tire quality grade) rating. Expected tread life is assigned a number ranging between 100 and 500.

This is listed relative to a baseline of 100. A 300-rated tire would be expected to last three times as long as a 100-rated tire. Many factors affect actual tire life, such as inflation pressure, wheel alignment, vehicle load, shock absorber condition and driving habits. Straight-line wet-surface traction is also assigned a letter, with A being the highest and C the lowest. The tire’s resistance to heat is the final letter, with A being the best and C the worst.

Careful scrutiny of a tire’s sidewall also will show the DOT compliance code and tire identification number. This indicates the tire meets the Department of Transportation’s safety standards, the manufacturer who made it and the build date. The first two letters indicate the manufacturer and country of origin, and the numbers that follow list the week and year of manufacture. The number 1104 would tell you the tire was made during the eleventh week of 2004.

Ready to start shopping? Start by checking the tire information sticker on your vehicle’s door-edge or jamb. It lists all the pertinent tire requirements for your vehicle.

To easily compare prices, specs and features on a wide variety of tires, check out Tirerack.com.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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AP-NY-04-23-04 0625EDT