Speedometer Error Calculator
See exactly how a tire size change affects your indicated vs actual speed — and whether you’re unknowingly speeding.
Speedometer Error Calculator
Stock vs New tire → actual speed
How It Works
Your speedometer reads off wheel rotations. If the new tyre is larger in diameter, more distance is covered per rotation, but the ECU calculates speed as if the original wheel diameter is still fitted.
How to Use This Calculator
- Enter your original (factory) tire code as Stock Tire.
- Enter the new tire code you have fitted or plan to fit.
- Enter a speed reading to convert — e.g. 100 km/h on the speedo.
- Click Calculate — see actual speed and percentage error.
- If error exceeds 2%, consider speedometer recalibration.
Worked Example
Reference Table
| Error % | At 100 km/h, actual | At 130 km/h, actual | Risk |
|---|---|---|---|
| +1% | 101 km/h | 131.3 km/h | Negligible |
| +2% | 102 km/h | 132.6 km/h | Acceptable |
| +3% | 103 km/h | 133.9 km/h | Near limit |
| −2% | 98 km/h | 127.4 km/h | Speedo reads high |
| +5% | 105 km/h | 136.5 km/h | Speedo reads dangerously low |
| −5% | 95 km/h | 123.5 km/h | Likely fail roadworthy check |
Frequently Asked Questions
Is it illegal to have an incorrectly reading speedometer?
In most countries the speedometer must not read below actual speed (showing you’re going faster than you really are is safer). EU rules allow up to +10% / 0% error. US FMVSS allows ±2 mph. Practically, keep error within ±3%.
Do taller tires always cause the speedometer to read low?
Yes — a larger diameter tyre covers more ground per wheel rotation than the calibrated original. The speed sensor still counts the same number of rotations per unit time, so the ECU reports the original (lower) speed.
Can I recalibrate the speedometer for new tire sizes?
Many modern ECUs allow speedometer correction via OBD-II tuning software or dealer programming. Older vehicles may need a mechanical or electronic speedometer correction gear/adapter in the drivetrain.
How do I know if my car has been fitted with non-standard tires?
Compare the tire sidewall code against the placard in the door jamb. If they differ, run this calculator to determine the actual error and correct your speed habits accordingly.
