1. Metric, or tire type, defines the proper use of the tire. For example, a "P" designation means that it is a passenger car tire. An "LT" designation is for a light truck.
2. Tire width is the width of the tire measured in mm from sidewall to sidewall. This tire width is 225 mm.
3. Aspect ratio is the ratio of the height of the tire's cross-section to its width. On our example, 60 means that the height is equal to 60% of the tire's width.
4. Construction tells you how the layers of the tire were put together. "R" stands for radial, which means the layers run radially across the tire. "B" stands for bias construction, which means that the layers run diagonally across the tire.
5. Rim diameter is the width in inches or cm of the wheel from one end to the other. The diameter of this wheel is 16 in. Always replace a tire on a rim with another tire of exactly the same rim diameter designation and suffix letters.
6. Load index is an assigned number that corresponds with the load-carrying capacity of a tire. Most passenger car tire load indexes range from 75 to 100, but a few carry more. You'll also find the maximum load elsewhere on the tire sidewall, both in lbs and kg.
7. Speed rating tells you the maximum service speed for a tire. A speed rating isn't, however, a recommendation to exceed speed limits, and doesn't indicate how well a tire handles or corners.
If you're wondering whether your car needs an alignment, first look at your tires. Uneven tire wear -- often, more wear on the outside of some tires -- is a prime indicator that your car is likely out of alignment. Here are a few more indicators:
- your car seems to be drifting to one side, even when you think you're driving straight
- your steering wheel vibrates
- you are driving straight, but your steering wheel isn't centered