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What Do The Numbers Mean?

P235/75 R 15 (example)

First Letter=Tire Type:

    There are many different types of tires, the most common are P for passenger car and LT for     light truck of SUV.

First Number=Tire Width:
        Measured in millimeters

Second Number=Aspect Ration:
    This number tells you the height of the tire in a percentage of the width measured in                 millimeters. IE; 75=75% of the width, the first number. 

Second Letter=Tire Construction;
    Most modern tires are R meaning they where built with construction.

Third Number=Rim Diameter

  • Never overinflated tires, only the inside of the tire will be contacting the road and you will lose traction and tire life.
  • Never under inflate for the same reasons. 
  • Use the recommend tire PSI by your vehicle, usually printed on the driver door.

http://www.offroaders.com/tech/AT-MT-Tires/tire-tech.htm#Tire_Type

Passenger car tires also have more information not found on the tire itself.
Find Info On Your Tires At http://auto.howstuffworks.com/framed.htm?parent=tire.htm&url=http://www.nhtsa.dot.gov

Table 1: Uniform Tire Quality Grading (UTQG)
Title
 Test
 Rating Best
 Rating Second
 Rating Third
Tread Wear
 Longevity Of Tread
 100+ Below
 Below
Traction Stopping Distance On Wet Concrete
 AA A B
Temperature Ability For Tire To Dissipate Heat
 A B C