To the uninitiated, the numbers an letters on the side of your tire may be only slightly less confusing than Greek alphabet soup. However, they're not difficult to decipher once you know the basics.
All tire codes will be in this configuration: 205/55 R 16 68 V, (possibly preceded by a P or LT, indicating passenger car or light truck respectively).
In the above example, "205" is the tire's width in millimeters and "55" is the aspect ratio. Aspect ratio (or "profile") is equal to the tire's sidewall high in percentage multiplied by the width (or 55 percent of 205 millimeters in this example, yielding a 112.75 mm tall sidewall).
The "R" in this example stands for "radial," which is the belt arrangement used in the tire. The only other marking you'll see is "B" for "bias ply," but bias plies are extremely uncommon on cars. The "16" indicates the diameter of the hole in the tire, which determines the size rim it will fit; 16 inches in this example.
The last 2-digit number (68 in this example) indicates the tire's load rating, and corresponds to a load in pounds on a load-rating chart. The last letter is the tire's speed rating; the further down the letter in the alphabet the higher the tire's speed rating. This example's "V" rating signifies it as a very fast high-performance tire.
