Tire & wheel tools
Speedometer error calculator
Enter the factory and the new tire size plus the speed on the dial to estimate the real speed, the difference and the percentage — with a reference table of common speeds.
Estimated true speed
61.8 mph
Diameter ratio +2.98%
Swapping 225/45R17 for 245/40R18 changes the diameter +2.98% — the speedometer drifts by the same share.
| Indicated | Estimated true |
|---|---|
| 10 mph | 10.3 mph |
| 20 mph | 20.6 mph |
| 30 mph | 30.9 mph |
| 40 mph | 41.2 mph |
| 50 mph | 51.5 mph |
| 60 mph | 61.8 mph |
| 70 mph | 72.1 mph |
| 80 mph | 82.4 mph |
The diameter change exceeds ±3%, so the speedometer error grows with it — and pressure, load and wear add more on top.
Computed from theoretical tire diameters only. Real rolling radius, pressure, load and wear add deviation — treat this as an estimate.
How it is calculated
True speed = indicated speed × new diameter ÷ factory diameter. The speedometer is calibrated to the factory diameter, so a taller tire covers more ground per revolution and the real speed runs higher than indicated.
The estimate uses theoretical size-code diameters. The real rolling radius is slightly less than half the diameter and shifts with pressure, load and wear — actual error can differ another 1–3 points.
How to fill each field
- Factory tireUse the size the car shipped with, not what is on it now — if the current tires are already non-stock, the dial reading is off to begin with.
- Indicated speedThe number on the dial. The common-speed table scales by the same diameter ratio.
- Speed unitSwitching km/h and mph converts the entered speed at 1mph = 1.609km/h.
FAQ
225/45R17 to 245/40R18 — how far off is the speedo?
The diameter ratio is 653.2÷634.3 ≈ 1.0298, so an indicated 60mph is about 61.8mph true — the car runs ~3% faster than the dial shows.
Why does a tire change skew the speedometer?
Speed is inferred from wheel revolutions × diameter, calibrated on the stock diameter. A taller tire travels further per revolution so the dial reads slow; a shorter one reads fast.
Can I use this for inspection or a speeding dispute?
No — this estimates from theory only and ignores the gauge's own error, rolling radius, pressure and wear. Use a GPS reading for an accurate figure.
Is the odometer affected too?
Yes, by the same ratio. A +3% diameter means an indicated 100 miles is really about 103 — the odometer undercounts.