Size

• A stamping on the side of every tire indicates its size. The stamping is formatted 195/60R15; the three numerical values are the size values and they change with every size. The first three numbers are the width of the tire in millimeters. The second set of numbers is called the aspect ratio. It is the sidewall height of the tire relative to the width. The last set of numbers is the diameter of the inner hole, where the rim goes.
  
  

Speed Rating

• Every tire goes through a complex test to rate it for the maximum speed it can handle for an extended time period. These ratings are defined by the letters Q, R, S, T, U, H, V, W and Y, abd they translate to 99, 106, 112, 118, 124, 130, 149, 168 and 186 mph, respectively.
  
  

Treadwear

• The treadwear rating is commonly misunderstood. The treadwear number is a three-digit code on the side of the tire from 100 through 900. The numbers are relative to themselves, that is with 100 considered the lowest rating, a tire with a 200 rating should last twice as long as a 100, a 400 should last twice as long as a 200, and so forth.