Measured Tire Rollout Calculator

This calculator works by dividing the distance a tire travels by the number of turns it makes. The answer is the tire's real rolling circumference under load.

1. Mark the tire tread and the ground at the bottom of the tire.
2. Roll the vehicle straight for 1, 2 or 5 full turns.
3. Measure the distance between ground marks, for example 10,250 mm over 5 turns.
4. Enter the distance and turns; read the rollout: 2,050 mm (80.7 in).

The formula is Rollout = Distance Traveled ÷ Number of Revolutions.

Worked example: 12,000 mm over 5 turns gives 2,400 mm (94.5 in). The effective diameter is 30.08 in and the tire turns 670.6 times per mile.

Measuring over 5 turns cuts the error from a 5 mm marking mistake to 1 mm per turn.

The conversion table converts 11 rollout values from 1,800 mm to 2,600 mm into effective diameter, revs per mile and revs per km. A 2,000 mm rollout equals 804.7 revs per mile and 500 revs per km.

Effective diameter from a loaded rollout is always smaller than the nominal diameter of the same tire.

The Measured Tire Rollout Calculator is a tool that turns a measured rolling distance into circumference, effective diameter and revolutions per distance. Rollout is the most accurate way to find a tire's real rolling size.

Weight flattens the tire at the contact patch, and tread blocks squirm. Both effects make the rolling circumference shorter than π × nominal diameter. Drag racers, rally crews and speedometer installers measure rollout for this reason.

This calculator has 4 uses: speedometer calibration, drag racing, rally timing and bike computers.

• Calibrate speedometers: Enter the measured rollout for exact readings.
• Set up drag cars: Use rollout for gear and ET predictions.
• Time rally stages: Calibrate trip meters to the real tire.
• Set bike computers: Enter rollout in mm on cycle computers.