Speedometer Error
Calculate speedometer error from tire changes
How to read a tire size (e.g. 225/65R17)
225
Width (mm)
65
Aspect Ratio (%)
R17
Rim Size (in)
Actual Speed
63.0 mph
Error: +3.0 mph (5.0%)
⚠️ Consider recalibrating your speedometer
Quick Facts
- •Larger tires = speedometer reads low
- •Smaller tires = speedometer reads high
- •Most states allow up to 5% error
Speedometer Accuracy
Your speedometer is calibrated for the original tire size. Larger tires mean you're going faster than indicated; smaller tires mean slower. A 3% change in tire diameter creates a 3% speedometer error. At 60 mph indicated, you'd actually be going 61.8 mph.
Understanding the speedometer error
A speedometer counts wheel rotations and multiplies by an assumed tyre circumference. Fit taller tyres and each rotation covers more ground, so you travel faster than the display shows. Fit shorter tyres and the opposite happens. The error is proportional to the change in overall diameter, which is why a plus size wheel with a lower profile tyre can be error free.
Worked examples
Error from moving 225/65R17 to 235/70R17
- 1Original diameter: 17 + 2 × (225 × 0.65 ÷ 25.4) = 17 + 11.52 = 28.52 in.
- 2New diameter: 17 + 2 × (235 × 0.70 ÷ 25.4) = 17 + 12.95 = 29.95 in.
- 3Ratio: 29.95 ÷ 28.52 = 1.0501, a 5.0 percent increase.
- 4At an indicated 70 mph, true speed is 70 × 1.0501 = 73.5 mph.
The speedometer reads 3.5 mph slow at 70, and the odometer under-records by 5 percent.
Correcting an odometer reading
- 1The trip meter shows 300 miles after the tyre change.
- 2Multiply by the same 1.0501 ratio.
- 3300 × 1.0501 = 315.0 actual miles.
- 4Fuel economy figures based on the raw reading are therefore 5 percent pessimistic.
315 real miles against 300 displayed.
Speedometer reading against true speed
| Diameter change | Indicated 30 | Indicated 60 | Indicated 70 |
|---|---|---|---|
| -3% | 29.1 | 58.2 | 67.9 |
| -1% | 29.7 | 59.4 | 69.3 |
| 0% | 30.0 | 60.0 | 70.0 |
| +1% | 30.3 | 60.6 | 70.7 |
| +3% | 30.9 | 61.8 | 72.1 |
| +5% | 31.5 | 63.0 | 73.5 |
| +8% | 32.4 | 64.8 | 75.6 |
Practical tips
- •Keep the overall diameter change within 3 percent to stay inside factory tolerance for ABS and stability control.
- •Most factory speedometers already read 1 to 3 percent high from new, since regulations allow over-reading but not under-reading.
- •Verify with a GPS speed app on a flat road before assuming a calculation is exact. Tyre pressure and wear both shift the rolling radius.
Frequently asked questions
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This calculator is for educational and reference purposes only. Always verify critical calculations independently.