Speedometer Correction Factor Calculator
Calculate the speedometer correction factor for a tire change. Get the multiplier, inverse and new revs per mile for recalibration.
Calculate a tire-size correction multiplier.
Speedometer Correction Factor Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Speedometer Correction Factor Calculator Works?
The Speedometer Correction Factor Calculator works by dividing the new tire diameter by the calibrated tire diameter to produce one multiplier. The tool uses 3 steps.
Enter both sizes
Type the calibrated and fitted sizes.
Divide diameters
The calculator divides the new diameter by the old diameter, for example 830.8 ÷ 802.8 = 1.0349.
Use the factor
Multiply any indicated speed or distance by 1.0349 to get true values, or enter the factor in a calibration tool that accepts a percent or multiplier.
Speedometer Correction Factor Calculator Formula
The Speedometer Correction Factor Calculator formula is CF = OD_new ÷ OD_cal.
CF = OD_new ÷ OD_calCF⁻¹ = OD_cal ÷ OD_newCal% = CF × 100RPM_new = RPM_cal ÷ CF- CF
- Multiplier from indicated to true values
- CF⁻¹
- Multiplier from true to indicated values
- RPM_cal
- Calibrated revs per mile
Worked example
A change from 225/45 R17 (634.3 mm) to 245/40 R18 (653.2 mm) gives CF = 1.0298. An indicated 100 km/h equals a true 103.0 km/h (64.0 mph), and an indicated 1,000 km equals a true 1,029.8 km (639.9 mi).
Speedometer Correction Factor Calculator Conversion Table
The Speedometer Correction Factor Calculator conversion table lists the correction factor, inverse factor and new revs per mile for 7 size changes.
| Size change | Correction factor | Inverse factor | Calibration percent | New revs per mile |
|---|---|---|---|---|
| 225/45 R17 → 225/40 R18 | 1.0046 | 0.9954 | 100.46% | 804 |
| 225/45 R17 → 245/40 R18 | 1.0298 | 0.9711 | 102.98% | 784 |
| 265/70 R17 → 285/70 R17 | 1.0349 | 0.9663 | 103.49% | 617 |
| 265/70 R17 → 285/75 R17 | 1.0704 | 0.9342 | 107.04% | 596 |
| 205/55 R16 → 195/55 R16 | 0.9826 | 1.0177 | 98.26% | 825 |
| 245/75 R16 → 265/75 R16 | 1.0388 | 0.9627 | 103.88% | 637 |
| 275/55 R20 → 275/65 R20 | 1.0679 | 0.9365 | 106.79% | 592 |
What is Speedometer Correction Factor Calculator?
A Speedometer Correction Factor Calculator is a tool that expresses a tire size change as a single multiplier that corrects speed and distance readings. One factor corrects speedometer, odometer, trip meter and fuel economy readings.
Calibration tools ask for the value in different forms. Some take revs per mile, some take tire diameter, some take a percent, and some take a multiplier. The calculator outputs all 4.
A factor above 1.0000 means the tire is taller than calibrated, and a factor below 1.0000 means the tire is shorter.
What are the uses of Speedometer Correction Factor Calculator?
There are 4 main uses of the Speedometer Correction Factor Calculator, including programmer input and spreadsheet corrections.
Programmer input
Enter the percent or revs per mile into a handheld tuner.
Spreadsheet corrections
Correct logged mileage with one multiplier.
Fleet telematics
Adjust telematics distance after a fleet-wide tire change.
Quick mental math
Multiply any dash value by the factor in one step.
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