Tire Size Calculations

Therefore, using the following values for example:

Speedometer Gear Ratio = 1000 revs/mile
Speedometer Pinion Gear Teeth = 17
Transmission Output Shaft Teeth = 7
Driveshaft Revs per mile = 1000 / 7 x 17
Driveshaft Revs per mile = 2428.571429

Axle Ring Gear Teeth = 41
Axle Pinion Teeth = 14
Axle Gear Ratio = 41 / 14
Axle Gear Ratio = 2.928571429 (ie, 2.93:1)

Tire Turns per mile = Driveshaft Revs / Axle Ratio
Tire Turns per mile = 2428.571429 / 2.928571429
Tire Turns per mile = 829.2682927

Required Circumference = 5280 feet/mile / Tire Turns per Mile
Required Circumference = 5280 / 829.2682927
Required Circumference = 6.367058824 ft

Required Tire Diameter = Required Circumference / Pi x (12 in/ft)

Required Tire Diameter = 6.367058824 / 3.141592654 x 12
Required Tire Diameter = 24.32"

However, the rolling (dynamic) diameter of a tire is about 3% smaller than the calculated static diameter (see Barry's Tire Tech - Tire Dimensions). This means we need a slightly larger tire to keep the odometer accurate. Therefore,

Corrected Required Tire Diameter = 24.32 / 0.97
Corrected Required Tire Diameter = 25.07"

The static diameter of the standard replacement radial-ply tire is the P175/80R13 and its diameter is:

Tire Diameter = rim diameter + 2 x the tire section height
Tire Diameter = 13" + 2 x (175 mm x 0.80) x (1" / 25.4 mm)
Tire Diameter = 24.02"

The P175/80R13, having a static diameter of 24.02", would cause the odometer to record 4.4% more distance than actual. A better choice for this vehicle would be the P185/80R13, which would have a static diameter of 24.65" and an odometer inaccuracy of 1.67%. This compares well with the original tire of the car used in this example (1965 Plymouth Barracuda) is a 6.50-13 bias-ply tire. Assuming it had an aspect ratio of 88%, it would have recorded 1.96% more miles than actually driven.