Tire Size Speedometer Calculator
When you install tires with a different overall diameter than the factory specification, your speedometer and odometer will read incorrectly because they are calibrated to the original tire's circumference. This calculator lets you enter the original (stock) tire diameter and your new tire diameter, along with the indicated speedometer reading, to find your actual true speed and the percentage error. Knowing this difference is important for legal driving, odometer accuracy, and understanding how tire changes affect your vehicle's performance characteristics.
Speedometer error formula
Actual Speed = Speedometer Reading * (New Diameter / Stock Diameter) Speedometer Error (%) = ((Actual - Indicated) / Indicated) * 100
A positive error percentage means your actual speed is higher than the speedometer shows (larger tires). A negative percentage means your speedometer over-reads (smaller tires than stock).
Converting a metric tire code to diameter
- Example: 265/60R17. Width = 265 mm. Aspect ratio = 60%.
- Sidewall height = 265 * 0.60 = 159 mm.
- Overall diameter = rim (17 * 25.4 = 431.8 mm) + 2 * 159 = 749.8 mm.
- Divide by 25.4 to convert to inches: 749.8 / 25.4 = 29.52 inches.
Frequently asked questions
Why does changing tire size affect the speedometer?
The speedometer reads speed based on wheel revolutions per unit time. A larger tire travels a greater distance per revolution, so the vehicle actually moves faster than the speedometer indicates. A smaller tire shows a higher reading than actual speed.
How do I find my tire diameter from the tire code?
For a tire coded 245/55R17: the section width is 245 mm, the aspect ratio is 55%, so sidewall height = 245 * 0.55 = 134.75 mm. Overall diameter = rim diameter in mm + 2 * sidewall = (17 * 25.4) + 2 * 134.75 = 431.8 + 269.5 = 701.3 mm = 27.61 inches.
How much speedometer error is legal?
NHTSA regulations (49 CFR 393.82) allow commercial vehicle speedometers to read within a range of actual speed. No federal rule governs passenger car speedometers, but most OEM speedometers show 1-3% over the true speed for safety. A 5-10% error from tire changes is common.
Can I fix speedometer error from a tire change?
Yes. Many late-model vehicles allow speedometer recalibration through dealer scan tools or aftermarket tuners. For older vehicles, a GPS speedometer or electronic speedometer recalibrator inserted in the VSS signal can correct the reading.
Does tire size change affect my odometer?
Yes. The odometer uses the same wheel revolution signal as the speedometer. A larger tire means the odometer reads fewer actual miles than you have traveled; a smaller tire records more miles than traveled. This affects fuel economy calculations and lease mileage tracking.
Official sources
- NHTSA: nhtsa.gov - vehicle safety standards including speedometer accuracy.
- NHTSA 49 CFR Part 393: ecfr.gov - speedometer regulations.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.