Speedometer Correction Factor Calculator
Get one number to fix speed and distance. Use the calculator below. Results update as you type.
How the Speedometer Correction Factor Calculator Works
This calculator works by dividing the new tire diameter by the old one to produce a single multiplier. The multiplier converts any indicated speed or distance into a true value.
- Enter the size the speedometer was set for, for example 265/70R17 (31.61 in).
- Enter the fitted size, for example 315/70R17 (34.36 in).
- Read the speed factor: 1.0872.
- Read the recalibration factor: 0.9198.
The Formula
The formula is Speed Factor = New Diameter ÷ Old Diameter.
Speed Factor = D_new ÷ D_old Recalibration Factor = D_old ÷ D_new True Speed = Indicated × Speed FactorWorked example: a 245/70R17 (30.50 in) to a 33 in tire gives 33.00 ÷ 30.50 = 1.0818. A recalibration tool expecting a reduction factor uses 0.9244.
Some programmers ask for the speed factor and others ask for the inverse. Check which one your device expects.
Conversion Table
The conversion table lists speed and recalibration factors for 8 tire changes. Speed factors run from 0.9992 (almost no change) to 1.0919 (255/70R18 to 35x12.50R18).
Multiply any indicated speed or odometer distance by the speed factor to get the true value.
| Original | New | Old Dia (in) | New Dia (in) | Speed Factor (×) | Recalibration Factor |
|---|---|---|---|---|---|
| 205/55R16 | 225/50R16 | 24.88 | 24.86 | 0.9992 | 1.0008 |
| 225/65R17 | 235/65R17 | 28.52 | 29.03 | 1.0179 | 0.9824 |
| 245/75R16 | 265/75R16 | 30.47 | 31.65 | 1.0388 | 0.9627 |
| 265/70R17 | 285/70R17 | 31.61 | 32.71 | 1.0349 | 0.9663 |
| 265/70R17 | 315/70R17 | 31.61 | 34.36 | 1.0872 | 0.9198 |
| 275/65R18 | 295/70R18 | 32.07 | 34.26 | 1.0681 | 0.9362 |
| 255/70R18 | 35x12.50R18 | 32.06 | 35.00 | 1.0919 | 0.9159 |
| 245/70R17 | 33x12.50R17 | 30.50 | 33.00 | 1.0818 | 0.9244 |
What Is the Speedometer Correction Factor Calculator?
The Speedometer Correction Factor Calculator is a tool that produces one multiplier for converting speedometer and odometer readings after a tire size change. The factor is a single number you can store and reuse.
Correction factors are the input format for many speedometer calibration tools, gauge clusters and data loggers. A factor of 1.05 means the vehicle travels 5% farther and faster than the instruments show.
Common Uses
This calculator has 4 uses: programmer input, data logging, mileage records and gauge setup.
- Program calibrators: Enter the factor in a handheld tuner.
- Correct data logs: Fix logged speed after the fact.
- Correct mileage records: Multiply odometer readings for logs.
- Set aftermarket gauges: Enter the factor in programmable gauges.