Speedometer Error Calculator
Calculate speedometer error after changing tire size. See true speed at 30, 60 and 100 mph or km/h and whether the dash reads fast or slow.
Estimate speed changes after tire resizing.
Speedometer Error Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Speedometer Error Calculator Works?
The Speedometer Error Calculator works by treating the percent change in tire diameter as the percent error of the speedometer. The calculation takes 3 steps.
Enter the calibrated size
Type the size the speedometer was set for, usually the factory size such as 265/70 R17.
Enter the new size
Type the fitted size, for example 285/75 R17.
Read the error
The calculator returns the error in percent and the true speed at common indicated speeds, with a note that a taller tire makes the dash read slow.
Speedometer Error Calculator Formula
The Speedometer Error Calculator formula is Error % = (OD_new − OD_cal) ÷ OD_cal × 100, where a positive value means the vehicle travels faster than indicated.
E% = (OD_new − OD_cal) ÷ OD_cal × 100V_true = V_ind × OD_new ÷ OD_calΔV = V_true − V_ind- OD_cal
- Diameter the speedometer is calibrated for (mm)
- OD_new
- Diameter of the fitted tire (mm)
- V_ind
- Indicated speed
Worked example
Going from 265/70 R17 (802.8 mm) to 285/75 R17 (859.3 mm) gives an error of +7.04%. At an indicated 60 mph, the truck travels 64.2 mph (103.4 km/h).
Speedometer Error Calculator Conversion Table
The Speedometer Error Calculator conversion table shows the speedometer error of 8 tire sizes on a truck calibrated for 265/70 R17.
| Fitted size | Error | True speed at 60 mph | True speed at 100 km/h | Dash reading |
|---|---|---|---|---|
| 265/65 R17 | -3.30% | 58.0 mph | 96.7 km/h | reads fast |
| 265/70 R17 | +0.00% | 60.0 mph | 100.0 km/h | accurate |
| 275/70 R17 | +1.74% | 61.0 mph | 101.7 km/h | reads slow |
| 285/70 R17 | +3.49% | 62.1 mph | 103.5 km/h | reads slow |
| 285/75 R17 | +7.04% | 64.2 mph | 107.0 km/h | reads slow |
| 315/70 R17 | +8.72% | 65.2 mph | 108.7 km/h | reads slow |
| 305/70 R18 | +10.14% | 66.1 mph | 110.1 km/h | reads slow |
| 35x12.50R17 | +10.74% | 66.4 mph | 110.7 km/h | reads slow |
What is Speedometer Error Calculator?
A Speedometer Error Calculator is a tool that predicts how far a speedometer reading drifts from true speed after a tire size change. The speedometer counts wheel or transmission rotations and assumes the factory tire circumference.
A taller tire covers more road per turn, so the speedometer reads lower than true speed. A shorter tire makes the speedometer read higher than true speed.
Factory speedometers already read 1% to 5% high in many markets. United Nations Economic Commission for Europe (UNECE) Regulation 39 forbids a speedometer from showing less than true speed, and a taller tire can push a car past that rule.
What are the uses of Speedometer Error Calculator?
There are 4 main uses of the Speedometer Error Calculator, including speed ticket avoidance and recalibration decisions.
Speed limit compliance
Know the true speed behind the dash reading after an upsize.
Recalibration decisions
Decide whether the error justifies reprogramming.
Inspection checks
Confirm the speedometer still meets local inspection rules.
Cruise control setting
Set cruise control to an indicated speed that matches the limit.
Related Speedometer & Distance Tools
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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.