Measured Tire Rollout Calculator
Calculate rolling circumference from a measured rollout. Convert distance and counted revolutions into loaded diameter and revs per mile.
Calculate circumference from measured travel.
Measured Tire Rollout Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Measured Tire Rollout Calculator Works?
The Measured Tire Rollout Calculator works by dividing a measured travel distance by the number of tire revolutions counted over that distance. The test takes 4 steps.
Mark the tire and floor
Set tire pressure to the placard value, put a chalk mark on the tread at the bottom and a matching mark on the floor.
Roll the vehicle
Push or drive the vehicle in a straight line for 5 to 10 full revolutions on a flat surface with normal load.
Measure the distance
Mark the floor where the chalk line returns to the bottom and measure the distance in meters or feet.
Enter the results
Enter the distance and the revolution count. The calculator returns rolling circumference, loaded rolling diameter and revs per mile.
Measured Tire Rollout Calculator Formula
The Measured Tire Rollout Calculator formula is C_roll = Distance ÷ Revolutions.
C_roll = D ÷ ND_eff = C_roll ÷ πRPM = 1,609,344 ÷ C_roll (mm)- D
- Measured distance (mm)
- N
- Counted full revolutions
- D_eff
- Effective rolling diameter under load (mm)
Worked example
A tire covers 19.9 m (65.3 ft) in 10 revolutions. The rolling circumference is 1,990 mm (78.3 in), the effective rolling diameter is 633.4 mm (24.94 in), and the tire turns 809 times per mile.
Measured Tire Rollout Calculator Conversion Table
The Measured Tire Rollout Calculator conversion table converts 6 sample rollout measurements into circumference, effective diameter and revs per mile.
| Measurement | Rolling circumference | Circumference (in) | Effective diameter | Revs per mile |
|---|---|---|---|---|
| 5 revs over 9.8 m | 1,960 mm | 77.2 in | 623.9 mm | 821 |
| 10 revs over 19.9 m | 1,990 mm | 78.3 in | 633.4 mm | 809 |
| 10 revs over 25.2 m | 2,520 mm | 99.2 in | 802.1 mm | 639 |
| 5 revs over 12.75 m | 2,550 mm | 100.4 in | 811.7 mm | 631 |
| 20 revs over 40.0 m | 2,000 mm | 78.7 in | 636.6 mm | 805 |
| 10 revs over 21.6 m | 2,160 mm | 85.0 in | 687.5 mm | 745 |
What is Measured Tire Rollout Calculator?
A Measured Tire Rollout Calculator is a tool that turns a physical rollout test into the true rolling circumference of a mounted, loaded tire. A rollout test measures the floor distance covered over a known number of wheel turns.
Rollout gives the most accurate number for speedometer correction, more accurate than the size code and more specific than a published figure. The measurement captures the vehicle's real weight, the actual pressure and the current tread depth.
Racing classes such as drag racing and karting use rollout values for gear selection and timing setups.
What are the uses of Measured Tire Rollout Calculator?
There are 4 main uses of the Measured Tire Rollout Calculator, including precise speedometer calibration and race setup.
Precise calibration
Program speedometers with measured rather than estimated circumference.
Race gearing
Pick final drive ratios from the true loaded rollout.
Worn tire checks
Measure how much rolling distance a worn tire has lost.
Pressure effects
Measure rollout at 2 pressures to see the effect of inflation.
Related Core Tire Dimensions Tools
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