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.

Q. What do all those numbers mean on the sidewall of the tire?

A. Here's an illustration that should help you gain some important knowledge about your tires.

3.  Aspect ratio.
The height of the sidewall section compared to the width of the tire.   Example:  If this number was a 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 the other 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.
In simple terms, this indicates the distance across the "doughnut hole" of a tire. You must match wheel diameter and the tire opening.  A tire with a 15 size designation will not work on a 14" wheel, and vice versa.  Improper 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 the car. Never choose a tire with a lower load carrying capacity than the original equipment tire size.

As for the speed symbol, it means the tire is speed-rated on an indoor test wheel to the following speeds:

A.   The 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. 

Let's look at the example of a tire with a UTQG of 300 AB.   The 300 is treadwear, which represents the tire's comparative wear when measured against the government-mandated tire which is rated at 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 possesses.

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 manufacturer determines UTQG rating for their products.   Realize that UTQG's are more accurate when comparing tires within the same line versus tires from two different makers.

Q.  Where should I buy tires?

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