Estimated actual speed only. This tool does not recalibrate vehicle electronics. Speedometer behaviour varies by vehicle.
How tire size affects speedometer readings
A vehicle estimates speed from rotational information. If the replacement tire has a different rolling circumference, it travels a different distance per revolution. This calculator compares theoretical tire diameters and estimates actual road speed for a selected indicated speed and a reference table.
Inputs explained
- Original tire size: the size used as the current speedometer reference.
- New tire size: the proposed replacement size.
- Indicated speed: the value displayed by the vehicle, entered in mph or km/h.
- Result units: change presentation only; the diameter ratio is unit-independent.
Formula and assumptions
Estimated actual speed equals indicated speed multiplied by new theoretical diameter divided by original theoretical diameter. A larger new diameter produces a higher estimated actual speed than the indicated value; a smaller diameter produces a lower estimate. The calculation assumes speed is directly proportional to tire circumference.
Worked example
If the new calculated diameter is 2% larger than the original and the speedometer indicates 60 mph, the simplified estimate is about 61.2 mph. The same ratio applies in km/h. This is an estimate based on nominal size, not a measurement of actual road speed.
Interpreting the results
- Actual-speed estimate: calculated road speed at the selected indicated speed.
- Error percentage: relative difference produced by the tire-diameter ratio.
- Reference table: applies the same ratio to several common displayed speeds.
- Distance effect: a circumference change can also affect distance accumulation, although vehicle logic varies.
Use cases and important limitations
Use this calculator while evaluating alternate tire sizes or explaining why a diameter change alters indicated speed. Real tire circumference depends on product dimensions, pressure, load and tread wear. A vehicle may apply factory calibration or use data from multiple sensors. This tool does not recalibrate electronics and must not be used as a substitute for a legal speed measurement or professional diagnostic procedure.
Frequently asked questions
Will a larger tire make the speedometer read low?
Under the simplified diameter-ratio model, yes: the vehicle covers more distance per revolution, so actual speed is higher than the indicated speed.
Does changing units change the error percentage?
No. The same diameter ratio applies in mph and km/h; only the displayed speed unit changes.
Can this tool recalibrate my speedometer?
No. It only estimates the effect. Recalibration methods are vehicle-specific and should follow manufacturer or qualified technician guidance.
Sources and further checks
- NHTSA tire-safety guidance
- Your vehicle owner manual and tire-information placard
- The wheel and tire manufacturers’ current technical specifications