Tire speedometer calculator
Use the original tire as A and the replacement as B. Calculate road speed, the reading for a target speed and distance correction.
Example settings — replace these with your own.
Enter a full metric or flotation code. XL, C and load/speed markings are accepted.
All results use A as the original calibration and B as the replacement.
- A · Diameter
- 32.99 in83.80 cm
- B · Diameter
- 33.46 in85.00 cm
- Circumference ratio B ÷ A
- 1.014320
A is the original speedometer and odometer calibration. The calculation keeps that calibration after fitting B. Check road-speed accuracy with an independent measurement.
Speed correction table
| Speedometer reading (mph) | Estimated actual speed (mph) | Difference (mph) |
|---|---|---|
| 20 | 20.29 | +0.29 |
| 30 | 30.43 | +0.43 |
| 40 | 40.57 | +0.57 |
| 50 | 50.72 | +0.72 |
| 60 | 60.86 | +0.86 |
| 70 | 71.00 | +1.00 |
| 80 | 81.15 | +1.15 |
| 90 | 91.29 | +1.29 |
How the calculation works
r = circumference B ÷ circumference A = diameter B ÷ diameter A
Actual speed = indicated speed × r
Required indicated speed = target actual speed ÷ r
Actual distance = indicated distance × r
Worked example
A: 275/60R20 (838 mm) → B: 285/60R20 (850 mm).
r = 850 ÷ 838 = 1.014320
Estimated actual speed: 60 × 1.014320 ≈ 60.859 mph
Speedometer reading for that target: 60 ÷ 1.014320 ≈ 59.153 mph
Estimated distance traveled: 100 × 1.014320 ≈ 101.432 mi
Inputs and assumptions
Dimensions come from the nominal size code. Model, wear, load and pressure affect the actual rolling circumference.
The size portion supplies the geometry. XL/C and load/speed markings stay in shared links. Check service ratings against the vehicle placard and exact tire model.
Method and data