Actual Speed Calculator
Convert indicated speedometer speed into actual road speed after a tire change. Results in mph and km/h with the speed gap at each step.
Estimate road speed from indicated speed.
Actual Speed Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Actual Speed Calculator Works?
The Actual Speed Calculator works by multiplying the indicated speed by the ratio of the new tire diameter to the calibrated tire diameter. The tool uses 3 steps.
Enter both sizes
Type the calibrated size and the fitted size.
Enter the dash speed
Type the speed shown on the speedometer, for example 65 mph.
Read actual speed
The calculator returns the actual road speed in mph and km/h and the gap from the dash reading.
Actual Speed Calculator Formula
The Actual Speed Calculator formula is V_actual = V_indicated × (OD_new ÷ OD_cal).
V_a = V_i × OD_new ÷ OD_calΔV = V_a − V_ikm/h = mph × 1.609344- V_i
- Indicated speed
- V_a
- Actual road speed
- OD_new ÷ OD_cal
- Diameter ratio
Worked example
A truck calibrated for 265/70 R17 runs 285/70 R17 tires, a diameter ratio of 1.0349. At an indicated 70 mph (112.7 km/h), the actual speed is 72.4 mph (116.6 km/h).
Actual Speed Calculator Conversion Table
The Actual Speed Calculator conversion table converts 8 indicated speeds into actual speed for a truck calibrated for 265/70 R17 and fitted with 285/70 R17.
| Indicated speed | Actual (mph) | Actual (km/h) | Gap |
|---|---|---|---|
| 25 mph (40 km/h) | 25.9 mph | 41.6 km/h | +0.9 mph |
| 35 mph (56 km/h) | 36.2 mph | 58.3 km/h | +1.2 mph |
| 45 mph (72 km/h) | 46.6 mph | 74.9 km/h | +1.6 mph |
| 55 mph (89 km/h) | 56.9 mph | 91.6 km/h | +1.9 mph |
| 65 mph (105 km/h) | 67.3 mph | 108.3 km/h | +2.3 mph |
| 70 mph (113 km/h) | 72.4 mph | 116.6 km/h | +2.4 mph |
| 75 mph (121 km/h) | 77.6 mph | 124.9 km/h | +2.6 mph |
| 80 mph (129 km/h) | 82.8 mph | 133.2 km/h | +2.8 mph |
What is Actual Speed Calculator?
An Actual Speed Calculator is a tool that converts the speed shown on the dashboard into the vehicle's real road speed after a tire size change. The answer depends only on the diameter ratio between the fitted and calibrated tires.
The speed gap grows with speed. A 3.5% error is 0.9 mph at 25 mph and 2.8 mph at 80 mph.
GPS speed gives a second check. GPS reads true ground speed when the vehicle drives straight at a steady speed on a flat road.
What are the uses of Actual Speed Calculator?
There are 4 main uses of the Actual Speed Calculator, including legal speed checks and GPS cross-checks.
Legal speed checks
Find the dash reading that matches a posted limit.
GPS cross-checks
Compare calculated speed with a GPS app reading.
Track day timing
Convert dash speed into true speed for lap analysis.
Towing limits
Respect trailer tire speed limits with a corrected speed.
Related Speedometer & Distance Tools
View All 134 Subtools →Estimate speed changes after tire resizing.
Estimate displayed speed at a given road speed.
Calculate a tire-size correction multiplier.
Tabulate indicated and estimated actual speeds.
Estimate distance recording differences.
Adjust indicated distance for tire changes.