HomeSpeedometer error
Speedometer error after a tire size change
A taller tire turns fewer times per mile, so a speedometer set up for the old size reads low and the odometer under-counts; a shorter tire does the opposite. Going from 265/70R17 to 285/70R17, an indicated 60 mph works out at 62.1 mph.
True speed is the indicated speed times the new tire's overall diameter divided by the old one's, so the error grows in step with speed. The calculator below uses each size's calculated diameter.
A speedometer that read true on the 265/70R17 reads low on the 285/70R17: an indicated 60 mph is a true 62.1 mph. The odometer records about 9,660 miles for every 10,000 you drive.
| Speedometer shows | True speed | Error |
|---|---|---|
| 30 mph | 31.0 mph | +1.0 |
| 45 mph | 46.6 mph | +1.6 |
| 60 mph | 62.1 mph | +2.1 |
| 70 mph | 72.4 mph | +2.4 |
| 80 mph | 82.8 mph | +2.8 |
These are size-code figures only. Whether a size fits, clears and carries your load depends on your vehicle and the tire, so check with a tire shop.
True speed for the most-searched swaps
Each row is the second size fitted to a speedometer set for the first, at an indicated 60 mph100 km/h, tallest change first.
| Swap | Diameter change | True speed | Odometer per 10,000 |
|---|---|---|---|
| 235/75R15 to 31x10.50R15 | +7.3% | 64.4 mph107.3 km/h | 9320 |
| 265/70R17 to 285/75R17 | +7.0% | 64.2 mph107.0 km/h | 9340 |
| 265/60R18 to 275/65R18 | +5.1% | 63.1 mph105.1 km/h | 9520 |
| 285/70R17 to 315/70R17 | +5.1% | 63.0 mph105.1 km/h | 9520 |
| 265/50R20 to 275/55R20 | +4.9% | 62.9 mph104.9 km/h | 9540 |
| 265/75R16 to 285/75R16 | +3.7% | 62.2 mph103.7 km/h | 9640 |
| 265/70R17 to 285/70R17 | +3.5% | 62.1 mph103.5 km/h | 9660 |
| 285/70R17 to 285/75R17 | +3.4% | 62.1 mph103.4 km/h | 9670 |
| 275/55R20 to 275/60R20 | +3.4% | 62.0 mph103.4 km/h | 9670 |
| 265/70R17 to 265/75R17 | +3.3% | 62.0 mph103.3 km/h | 9680 |
| 275/60R20 to 295/60R20 | +2.9% | 61.7 mph102.9 km/h | 9720 |
| 275/65R20 to 35x12.50R20 | +2.7% | 61.6 mph102.7 km/h | 9740 |
| 315/70R17 to 35x12.50R17 | +1.9% | 61.1 mph101.9 km/h | 9820 |
| 265/70R17 to 275/70R17 | +1.7% | 61.0 mph101.7 km/h | 9830 |
| 275/65R18 to 285/65R18 | +1.6% | 61.0 mph101.6 km/h | 9840 |
| 275/60R20 to 285/60R20 | +1.4% | 60.9 mph101.4 km/h | 9860 |
| 275/55R20 to 285/55R20 | +1.4% | 60.8 mph101.4 km/h | 9870 |
| 285/70R17 to 33x12.50R17 | +0.9% | 60.5 mph100.9 km/h | 9910 |
| 305/70R18 to 35x12.50R18 | +0.5% | 60.3 mph100.5 km/h | 9950 |
| 245/75R17 to 265/70R17 | +0.4% | 60.3 mph100.4 km/h | 9960 |
| 285/75R16 to 305/70R16 | -0.1% | 60.0 mph99.9 km/h | 10010 |
| 235/85R16 to 265/75R16 | -0.2% | 59.9 mph99.8 km/h | 10020 |
| 255/75R17 to 265/70R17 | -1.4% | 59.2 mph98.6 km/h | 10140 |
| 275/70R18 to 285/65R18 | -1.7% | 59.0 mph98.3 km/h | 10180 |
Sources
- BFGoodrich: How to read a tire sidewall, retrieved 29 October 2026
- NIST Guide to the SI (SP 811), Appendix B.8: Factors for units listed alphabetically, retrieved 29 October 2026
- Performance Plus Tire: What is a 285/70R17 equivalent to?, retrieved 29 October 2026
- Performance Plus Tire: 285/70R17 instead of 265/70R17 (6 Oct 2026), retrieved 29 October 2026
- Performance Plus Tire: What is a 33x12.50R17 equivalent to? (26 Jun 2026), retrieved 29 October 2026
- SimpleTire: 285/70R17 tire size, retrieved 29 October 2026
- tiresize.com: 285-70R17 size page, retrieved 29 October 2026
- Toyo Tires: tire finder by size, retrieved 29 October 2026
Data verified 29 October 2026 against these sources. See the methodology for how disagreements are resolved.