Speedometer Error from Tire Size Calculator
Your speedometer assumes the diameter of the tires the vehicle left the factory with. Fit a taller or shorter tire and every reading is off by the ratio of the diameters. Enter the original and new tire diameters and your indicated speed, and this calculator returns your true speed, the speed error, and the percentage error. A larger tire makes the dial read low; a smaller tire makes it read high. The odometer is off by the same percentage.
Speedometer error formula
Diameter ratio = new diameter / original diameter
True speed = indicated speed * diameter ratio
Speed error = true speed - indicated speed
Error (%) = (diameter ratio - 1) * 100
The speedometer reads as if the original tire is fitted, so true speed scales by the diameter ratio. A ratio above 1 (larger tire) means you are actually faster than indicated.
What the result means
- A larger new tire gives a positive error: you are travelling faster than the dial shows.
- A smaller new tire gives a negative error: you are travelling slower than indicated.
- The odometer is off by the same percentage as the speed.
- Keep diameter changes near 3 percent or less to limit error.
- Many vehicles can be recalibrated to restore accurate readings after a tire change.
Speedometer error: frequently asked questions
How does changing tire size affect the speedometer?
A speedometer counts wheel rotations and assumes a fixed tire diameter to convert them to speed. Fitting a tire with a different diameter changes the distance covered per rotation, so the reading no longer matches reality. A larger diameter makes the speedometer read lower than your true speed; a smaller diameter makes it read higher.
How do you calculate true speed after a tire change?
True speed equals the indicated speed times the ratio of the new tire diameter to the original tire diameter: true speed = indicated speed times (new diameter / original diameter). If the new tire is larger, the ratio is above 1 and you are actually going faster than the dial shows.
What is an acceptable speedometer error?
Vehicle speedometers are designed never to read below true speed and are allowed a margin above it under standards such as the federal motor vehicle requirements; a small percentage error is normal even with stock tires. As a practical guide, keeping a tire diameter change within about 3 percent keeps the resulting speedometer error minor.
Does the odometer change too?
Yes. The odometer derives distance from the same wheel-rotation count, so a tire diameter change scales recorded distance by the same factor as speed. A larger tire under-records mileage and a smaller tire over-records it, by the same percentage as the speedometer error.
Can a shop recalibrate for new tire sizes?
Many modern vehicles can be recalibrated through the engine or instrument control module to account for a different tire diameter or final-drive ratio, restoring accurate speed and distance readings. Some vehicles need a programmer or dealer tool. Use this calculator to quantify the error first so you know whether recalibration is worth it.
Official sources
- National Highway Traffic Safety Administration: NHTSA vehicle standards and tire safety.
- Electronic Code of Federal Regulations: eCFR motor vehicle safety standards.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.