Estimated actual speed
103.0 km/h
Higher than indicatedAt indicated 100.0 km/h, assuming the original tire was the calibration reference.
Speedometer Error Calculator
At your selected indicated speed, estimate actual speed after a tire-size change. Keep the calibration assumption visible and compare a reference-speed table.
Result updated. At indicated 100.0 km/h, estimated actual speed is 103.0 km/h. The nominal diameter change is +2.98%; this does not recalibrate the vehicle.
Result
At indicated 100.0 km/h, estimated actual speed is 103.0 km/h. The nominal diameter change is +2.98%; this does not recalibrate the vehicle.
Speed difference
3.0 km/h
Higher actual speedNominal diameter ratio changes by +2.98%.
Details
- Original / proposed diameter
- 634.3 mm / 653.2 mm
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| Indicated | Estimated actual | Difference |
|---|---|---|
| 0.0 km/h | 0.0 km/h | 0.0 km/h |
| 20.0 km/h | 20.6 km/h | +0.6 km/h |
| 40.0 km/h | 41.2 km/h | +1.2 km/h |
| 60.0 km/h | 61.8 km/h | +1.8 km/h |
| 80.0 km/h | 82.4 km/h | +2.4 km/h |
| 100.0 km/h | 103.0 km/h | +3.0 km/h |
| 120.0 km/h | 123.6 km/h | +3.6 km/h |
Results use nominal geometry and the stated conventions. They are dimensional answers, not a fitment, safety or legality check.
How does tire size change actual speed?
Estimated actual speed equals indicated speed multiplied by proposed diameter divided by calibration-reference diameter. The ratio assumes the original speedometer calibration was accurate and is not a gauge certification.
The current tire input should represent the speedometer's calibration reference, not simply whatever tire happens to be installed if it was already changed. The estimate assumes the gauge was accurate on that reference size.
Enter the replacement size and the indicated speed shown on the gauge. Choose km/h or mph consistently. Zero speed returns zero actual speed even when the tire diameters differ; it does not mean the geometric percentage has become zero.
The relationship behind the estimate
Estimated actual speed = indicated speed × new nominal diameter ÷ original nominal diameter. Circumference has the same ratio because π cancels. The calculation uses the shared tire engine, so its diameter values agree with Tire Size and the full fitment tool.
A larger diameter covers more theoretical road distance per rotation and gives a higher estimate. A smaller diameter gives a lower one. The signed speed difference is actual minus indicated; the reference table applies the identical ratio at several indications.
A calculation is not recalibration
The table does not change vehicle electronics or prove real road speed. Factory gauge bias, the vehicle's sensing method, tire deflection, wear and loaded rolling circumference can all affect a real measurement.
If speedometer accuracy matters, follow the vehicle manufacturer's recalibration guidance and use an appropriate measured reference. This tool is not a method for choosing a safe road speed or bypassing speed limits.
Example: estimated speed at an indicated 100 km/h
Inputs: Current tire: 225/45R17; Proposed tire: 245/40R18; Indicated speed: 100 km/h. Metric results below are computed by the same engine as the live tool.
At indicated 100.0 km/h, estimated actual speed is 103.0 km/h. The nominal diameter change is +2.98%; this does not recalibrate the vehicle.
Swipe to view more, if needed
| Measure | Calculated value | Meaning |
|---|---|---|
| Estimated actual speed | 103.0 km/h | At indicated 100.0 km/h, assuming the original tire was the calibration reference. |
| Speed difference | 3.0 km/h | Nominal diameter ratio changes by +2.98%. |
Important limitations
This is a nominal-diameter estimate under an explicit original-tire calibration assumption. It does not recalibrate the vehicle, certify speed accuracy or replace manufacturer rolling-circumference data.
See the calculation methodology and testing and accuracy for precision, unit conventions and the distinction between nominal geometry and physical measurement.