Speedometer Calibration Calculator
Compare GPS vs dash speed, estimate changes from new tire diameters, then match exact sizes using dedicated speed calibration charts. Start with your tire size math from the tire size calculator to keep values consistent.
Speedometer Calibration Calculator
Use GPS readings or tire size changes to understand how far off your speedometer is.
GPS / Radar Comparison
GPS Cal Difference
3.33%
Speedometer reads slow
Calibrate by Tire Size Change
Stock Tire Specs
New Tire Specs
Diameter Difference
4.93%
New setup reads slower than actual speed
Stock Diameter
25.97"
660 mm
New Diameter
27.25"
692 mm
GPS Based Calibration Table
Use this to re-mark the gauge or set electronic corrections.
| Indicated (mph) | Actual (mph) | Diff |
|---|---|---|
| 20.0 | 20.7 | 0.67 mph |
| 30.0 | 31.0 | 1.00 mph |
| 40.0 | 41.3 | 1.33 mph |
| 50.0 | 51.7 | 1.67 mph |
| 60.0 | 62.0 | 2.00 mph |
| 70.0 | 72.3 | 2.33 mph |
| 80.0 | 82.7 | 2.67 mph |
| 90.0 | 93.0 | 3.00 mph |
| 100.0 | 103.3 | 3.33 mph |
| 110.0 | 113.7 | 3.67 mph |
| 120.0 | 124.0 | 4.00 mph |
Tire Size Based Calibration Table
Show the speed impact when your rolling diameter changes.
| Indicated (mph) | Actual (mph) | Diff |
|---|---|---|
| 20.0 | 21.0 | 0.99 mph |
| 30.0 | 31.5 | 1.48 mph |
| 40.0 | 42.0 | 1.97 mph |
| 50.0 | 52.5 | 2.46 mph |
| 60.0 | 63.0 | 2.96 mph |
| 70.0 | 73.4 | 3.45 mph |
| 80.0 | 83.9 | 3.94 mph |
| 90.0 | 94.4 | 4.43 mph |
| 100.0 | 104.9 | 4.93 mph |
| 110.0 | 115.4 | 5.42 mph |
| 120.0 | 125.9 | 5.91 mph |
Verify GPS on a straight road
Hold a steady speed on level ground and note the difference between your dash and GPS readings. Repeat twice for accuracy.
Correct tire circumference in ECUs
Many trucks and EVs allow custom tire diameter entry. Use the tire size percentage shown above when adjusting.
Recheck after any hardware change
Wheel and tire changes, gear swaps, or axle ratio mods all alter the final drive. Re-run this calibration after every mod.
1. Record a baseline
Use a GPS speed app or roadside radar sign at steady highway speed, then enter both readings above.
2. Measure rolling diameter
Input your stock and new tire sizes to capture the exact percent difference that throws the gauge off. Use the tire size calculator if you need baseline dimensions.
3. Apply the correction
Update your ECU, SpeedoHealer, tuner, or mark the cluster using the calibration tables that match your setup.
How to use these speed calibration charts
Start from your wheel
The wheel-size view mirrors the reference dataset from Speedometer Calibration Tire Size Charts by Wheel Size. Expand the section that matches your rim diameter (13" through 22" and specialty off-road sizes) and pick the tire code that lands closest to your required overall diameter.
Or start from target diameter
When you already know the rolling diameter you need—perhaps to correct for gear swaps—jump into the companion Speed Calibration Tire Size Charts by Diameter. Each block shows every compatible wheel size plus its tire size list for that exact diameter band.
Lock in legal compliance
After choosing a tire, feed its specs back into the calculator above to confirm your speedometer error stays within ±3% (the tolerance most inspection stations allow in North America and the EU).
Double-check mileage and ABS
Mileage, transmission shift points, and ABS/traction systems rely on the same rolling diameter. Match front and rear rolling radius and recalibrate any controllers so the ECU knows the new tire circumference.
Disclaimer
The calculations and recommendations on this site are for informational purposes only. Always consult your vehicle manufacturer, tire professional, and local regulations before making any changes to tire or wheel size.