Speedometer Error from Tire Size Calculator
Fitting tires with a different overall diameter than the originals throws off your speedometer and odometer, because both are calibrated to the original rolling diameter. This calculator takes your original and new tire diameters, plus the speed shown on the dial, and reports your true road speed and the percentage error. Use it after a tire upsize or downsize to know whether you are really going faster or slower than the needle reads.
Speedometer error formula
True speed = indicated speed * (new diameter / original diameter)
Error (%) = ((new diameter / original diameter) - 1) * 100
A positive error means you are actually travelling faster than the dial shows (larger tire). A negative error means slower (smaller tire). The odometer error has the same magnitude and sign.
Worked example
Original tire 25.0 inches, new tire 26.0 inches, dial reading 60 mph. The ratio is 26.0 / 25.0 = 1.04, so true speed is 60 * 1.04 = 62.40 mph, and the error is +4.00%. You are going 2.4 mph faster than the speedometer indicates.
Speedometer error: frequently asked questions
Why does changing tire size affect the speedometer?
A speedometer is calibrated for the original tire's rolling diameter, counting wheel revolutions and converting them to speed. A larger diameter tire travels further per revolution, so at the same indicated speed you are actually going faster than the dial shows. A smaller tire makes you go slower than indicated.
How do I find my tire's diameter?
Diameter can be computed from the sidewall size. For a tire marked like 225/45R17, diameter equals the wheel diameter in inches plus twice the section height, where section height is width (mm) times aspect ratio divided by 100, converted to inches. This calculator takes the two overall diameters directly so you can enter measured or computed values.
Does a larger tire make my odometer over-read or under-read?
A larger diameter tire makes the odometer under-read: it records fewer miles than you actually travel, because the wheel turns fewer times per mile than the calibration assumes. A smaller tire causes the odometer to over-read. The percentage error is the same as the speedometer error.
Sources
- The relation follows directly from circumference being proportional to diameter; both speed and distance scale with the diameter ratio. See NIST on length units: NIST SI Units.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.