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Tire Information
by Donald Ward aka ProBear

Below are listed several formulas that people have asked about when they change tire sizes:

 (1)   For figuring actual tire diameter use the following formula:

(Section Width X Aspect Ratio X 2) + Rim Diameter = Tire Diameter

                                  25.4

       (2)   For figuring Rotations per mile use the following formula:

                 5280 X 12         = Number of revolutions per mile

Tire Diameter  X 3.1415

The short version of the formula is as follows:

20,168                   =  Number of revolutions per mile

Tire Diameter

       

(3)   To figure actual MPH with different size tires uses the following formula: (This assumes that the MPH was accurate with the old tires.)

 New Tire Diameter     X   Speedometer MPH   = Actual MPH

Old Tire Diameter

  

(4)   To figure an effective axle ratio change, use the following formula:

 New Tire Diameter     X   Old Axle Ratio   = New Effective Axle Ratio

Old Tire Diameter

  

To Read the Sidewall of a tire, the following rules apply:

 The first 3 numbers are the width of the tire in millimeters.

The next 2 numbers represent the aspect ratio (Sidewall height divided by tread        width)

The next letter is usually a ‘R’, which stands for radial.

The next 2 numbers are the diameter of the wheel in inches.

On ''''P'''' metric tires, the final numbers and letter are the numerical code associated with the maximum load a tire can carry (called a load index, ranging from 0 to 254), followed by the letter representing the speed rating.

For SD tires, the load range is represented by letters ranging from ''''A'''' to ''''E'''', with ''''E'''' being the heaviest rating.

Also located on the sidewall are the Uniform Tire Quality Grading Standards. The following represents them:

 Treadwear - This number is an indicator of the theoretical life of the tire. Numbers assigned are typically from 60 to 620, shown in 20-point increments. The higher the number, the longer the tire will last. This is a hypothetical number meant only to represent a comparison of tires under similar, normal conditions.

 Traction – A letter representing the straight line braking traction on wet pavement. Letters A, B, or C are assigned, with A the best, and C the worst.

Temperature – Similar three letter grading system representing the tires ability to withstand heat. Once again, A is the best, and C is the worst.

The other marking the people are concerned about is the DOT number. Every tire has these markings, and usually they are located on the face of the tire toward the underside of the vehicle.

The first letter and number after the DOT will designate the tire’s manufacturer and then the plant code with the last three numbers representing the date of manufacture of the tire.