Speedometer Error Calculator
Calculate speedometer error and true speed when changing tire sizes.
How to use this tool
- Use the tire diameter calculator to find your original and new tire diameters.
- Enter both diameters and your speedometer reading.
- The result shows your true speed and the percentage speedometer error.
Find out how much your speedometer is off after swapping to a different tire size.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Actual speed (MPH) = Indicated speed × New tire diameter ÷ Old tire diameter
Speedometer error (%) = (New diameter − Old diameter) ÷ Old diameter × 100
How it works
A vehicle's speedometer reads from wheel rotation speed, which is calibrated to the original tyre diameter. When a larger or smaller tyre is fitted, the wheel travels a different distance per revolution while the speedometer still assumes the original diameter. This calculator finds the true road speed and the percentage by which the speedometer over- or under-reads. Results apply equally to odometer error since mileage is derived from the same wheel-speed signal.
Worked example
- Original tyre diameter = 24.97 in; new tyre diameter = 25.5 in; indicated speed = 60 MPH.
- Actual speed = 60 × 25.5 ÷ 24.97 = 61.27 MPH.
- Speedometer error = (25.5 − 24.97) ÷ 24.97 × 100 = 0.53 ÷ 24.97 × 100 ≈ 2.12%.
- The speedometer reads 60 but you are actually travelling at 61.27 MPH.
Actual speed = 61.27 MPH; Speedometer error = +2.12%
Common mistakes to avoid
- Assuming the error is negligible for small changes -- a 3% diameter difference means the speedometer reads 60 MPH when you are actually doing 61.8 MPH.
- Comparing tires using only section width without recalculating overall diameter, since a wider tire with a lower aspect ratio can have nearly the same overall diameter.
- Ignoring the effect on odometer accuracy, which also affects oil-change intervals, lease mileage limits, and fuel economy calculations.
Key terms
- Indicated speed
- The speed shown on the vehicle's speedometer, which may differ from true speed when non-standard tyres are fitted.
- Actual speed
- The true road speed calculated from the new tyre's larger or smaller rolling circumference.
- Speedometer error
- The percentage difference between the speedometer reading and actual vehicle speed caused by a change in tyre diameter.
- Odometer error
- Cumulative mileage recording error that results from the same tyre-diameter mismatch affecting the speedometer — a 2% speedometer error also means a 2% odometer error.
Frequently asked questions
- Does changing tire size affect the speedometer?
- Yes. A larger-diameter tire covers more ground per revolution, so the speedometer reads low — you're actually going faster than it shows. A smaller tire makes it read high.
- How much tire size difference is acceptable?
- Most states allow up to 3% speedometer error. Keep tire diameter within 3% of stock to avoid odometer, ABS, and traction control issues.