Speedometer Error Calculator
Enter your original and new tire size to see the exact speedometer drift caused by the diameter change. Useful before plus-sizing or fitting a smaller spare.
Original tire
205/55 R16
New tire
225/45 R17
Original diameter
631.9 mm
New diameter
634.3 mm
Diameter difference
+2.4 mm
Difference %
+0.38%
Indicated speed
100 km/h
Actual speed
100.4 km/h
Speedometer error
+0.38%
Verdict
excellent
Why diameter changes the reading
A speedometer measures wheel rotations and multiplies by the factory-set circumference. If your new tire is 2% larger, every rotation now covers 2% more ground — so when the dashboard reads 100 km/h you are actually doing about 102 km/h. Going smaller flips the sign: indicated speed exceeds true speed.
The same signal feeds ABS, traction control and the odometer, so the error compounds across the vehicle. Within ±3% the system stays inside its design tolerance; beyond that, recalibration or a different size choice is the safer path.