Speedometer Error Calculator
See how a new size changes what the speedometer shows. Use the calculator below. Results update as you type.
Actual Speed = Indicated Speed × (New Diameter / Old Diameter)How the Speedometer Error Calculator Works
This calculator works by comparing the new tire's diameter with the original and applying the same percentage to the speedometer reading. A tire 3% taller makes the speedometer read 3% low.
- Enter the original size, for example 245/75R16 (30.47 in).
- Enter the new size, for example 265/75R16 (31.65 in).
- Read the diameter change: +3.88%.
- Read the error: an indicated 60 mph is a true 62.3 mph (100.3 km/h).
A positive change means the vehicle goes faster than the dial shows. A negative change means it goes slower.
The Formula
The formula is Actual Speed = Indicated Speed × (New Diameter ÷ Old Diameter).
Actual Speed = Indicated Speed × (New Diameter ÷ Old Diameter) Error (%) = (New Diameter − Old Diameter) ÷ Old Diameter × 100 Error (mph) = Actual Speed − Indicated SpeedWorked example: a −2% diameter change at an indicated 100 km/h (62.1 mph) gives a true 98.0 km/h, an error of −2.0 km/h.
The formula assumes the speedometer was accurate with the original tires. Many factory speedometers already read slightly high by design.
Conversion Table
The conversion table shows true speed at an indicated 60 mph and 100 km/h for 12 diameter changes from −6% to +10%. Every 1% change moves true speed by 0.6 mph at 60 mph and 1.0 km/h at 100 km/h.
Use the row that matches your diameter change. A +6% change means 63.6 mph when the dial shows 60.
| Diameter Change | True mph at 60 | Error (mph) | True km/h at 100 | Error (km/h) |
|---|---|---|---|---|
| -6% | 56.4 | -3.6 | 94.0 | -6.0 |
| -4% | 57.6 | -2.4 | 96.0 | -4.0 |
| -3% | 58.2 | -1.8 | 97.0 | -3.0 |
| -2% | 58.8 | -1.2 | 98.0 | -2.0 |
| -1% | 59.4 | -0.6 | 99.0 | -1.0 |
| +1% | 60.6 | +0.6 | 101.0 | +1.0 |
| +2% | 61.2 | +1.2 | 102.0 | +2.0 |
| +3% | 61.8 | +1.8 | 103.0 | +3.0 |
| +4% | 62.4 | +2.4 | 104.0 | +4.0 |
| +6% | 63.6 | +3.6 | 106.0 | +6.0 |
| +8% | 64.8 | +4.8 | 108.0 | +8.0 |
| +10% | 66.0 | +6.0 | 110.0 | +10.0 |
What Is the Speedometer Error Calculator?
The Speedometer Error Calculator is a tool that predicts how much a tire size change shifts the speedometer reading. The speedometer counts wheel or output-shaft turns and multiplies by the factory tire's circumference.
A taller tire covers more ground per turn, so the vehicle travels faster than the speedometer shows. A shorter tire does the opposite. The size of the error in percent equals the size of the diameter change in percent.
Common Uses
This calculator has 4 uses: ticket avoidance, recalibration decisions, inspection checks and cruise-control setup.
- Avoid speeding tickets: Know true speed after an upsize.
- Decide on recalibration: See if the error justifies a programmer.
- Pass inspections: Check speedometer accuracy rules in your region.
- Set cruise control: Pick a set speed that gives the true speed you want.