The right tire size matters. The first place to start is your car's owner's manual. If you don't have it, you can easily find the manufacturer-recommended size in these ways:

• On the driver's side door jamb
• Inside the glove compartment door
• Within the gas tank hatch

You can also find tire size on the sidewall of your current tires, assuming they're the proper size.
 

Letters

For most vehicles, the letter “P” indicates a passenger vehicle. “LT” specifies a light truck. And “T” and “ST” stand for temporary and special trailer.

Width

The first number is your tire's width, measured in millimeters.

Aspect Ratio

The number after the slash tells you how tall your tire's profile is.

Construction

The letters “R” or “D” refer to the internal structure and stability.

Rim Diameter

The next number is the wheel's diameter, measured in inches.

Load Index

The next number indicated how much weight your tire can support.

Speed Rating

The final figure is the maximum speed your tire can handle, indicated by a letter.