When you answer these questions, you narrow the choices to ones that fit your specific needs and wants. It helps the salesperson too. He or she won't waste your time discussing tires you don't want.

Determining Your Correct Tire Size
A. Look in your automobile owner's manual. You'll find the size fitted on the car originally. Unless you've changed wheels, that's the recommended size.
B. The tire size is written on the tire's sidewall. Here's and example of the way tire sizing looks on the sidewall:

1. P indicates a passenger (car-type) tire.
Other options would be no P indicating metric sizing (essentially the same as P-sizing, which has its heritage in Europe) or LT for light truck. LT truck tires are designed for heavier loads and more rugged service conditions.

2. Width of the tire, in millimeters.
The higher the number the wider the tire.

3. Aspect ration
The height of the sidewall section compared to the width of the tire. Example: – If this number was 50, then the tire section is half as tall as it is wide. Short sidewalls deliver crisp handling. Tall sidewalls give a smoother ride. For a specific tire width, the smaller this number, the shorter the sidewall.

4. Construction
R indicates radial construction. Unless you specify theater option, bias (which would have a D {diagonal} or B {belted bias} in this position instead of an R), you are purchasing a radial tire.

5. Wheel Size Designation
this indicates the distance across the "doughnut hole" of a tire, in simple terms. You must match wheel diameter and tire diameter. For example, a 15-inch diameter tire must only be mounted on a 15-inch diameter rim. A tire with a 15 size designation will not work on a 14" wheel, and vice versa. Improperly matching of wheel and tire size can cause serious injury or death during installation.

6. Service Description
Includes the load index (a number) and a speed symbol (a letter). About load index, remember: Keep the number the same as or greater than the original tires' load index. That number stands for a specific weight-carrying capability. Key is that the higher the number, the greater load-carrying capacity. So, keep the number the same or higher, because your tires are matched to the weight of your car. Never choose a tire with a lower load-carrying capacity than the original equipment tire size.

Now the speed symbol. It means the tire is speed rated on an indoor test wheel to:

We recommend that the replacement tires match the speed symbol of the original tires because these match the speed capability of the vehicle.

Uniform Tire Quality Grading (UTQG)

Uniform Tire Quality Grading (UTQG) is a system developed by the Department of Transportation which helps consumers compare tires in the areas of treadwear traction and temperature. Here's an example: a tire with a UTQG of 300 AB. The 300 s treadwear, which represents the tire's comparative wear when measured against the government-mandated tire which is rated 100. So, this tire wore three times longer than the control tire. Because of variations in operating conditions, the treadwear grade cannot be used to predict actual wear out mileage; it can be used to predict the relative wear rate of tires with different UTQG wear rates. Traction and temperature are graded A (superior), B (good), or C (average). Traction indicates the level of grip the tire posseses.

Temperature rating deals with the tire's ability to disperse built-up heat. Heat causes rubber to deteriorate over time, so dispersing heat increases tire life.

Each tire manufacturer determines UTQG ratings for their products. Realize that UTQGs are more accurate when comparing tires within the same line versus tires from two different makers.

Where Should I Buy Tires?
Buy your tires where you feel comfortable: a place you feel will take care of you during and after the sale. Because tires may have warranties as high as 80,000 miles or more, the period after the sale may be several years. Tire professionals help you maximize your investment and get the right tire for your driving needs. Buy your tires where the sales force helps your, not just sells you. In short, buy their reputation as well as their tires.