Quick Answer
- Speedometer error equals the change in overall tire diameter. 265/70R17 to 285/70R17 is 1.10 in taller (+3.5%), so the speedometer reads low: an indicated 60 mph is 62.1 mph (calculated).
- The odometer is off by the same percentage: 100 indicated miles are 103.5 actual miles.
- Continental says to keep rolling circumference within 3% of the original; a larger deviation can affect speedometer accuracy, ABS and stability control.
① Original Tire
→ 265/70R17
② Compare Tire
→ 285/70R17
Enter tire specs above and click Calculate to compare
Supports P-metric (225/65R17) and Imperial/flotation (31x10.5R15) formats
True Speed by Diameter Change
Positive means a taller tire, so the speedometer reads low. Calculated: true speed = indicated speed x (1 + change).
| Diameter change | 30 mph indicated | 50 mph indicated | 60 mph indicated | 70 mph indicated | 80 mph indicated | 100 km/h indicated |
|---|---|---|---|---|---|---|
| -5% | 28.5 | 47.5 | 57.0 | 66.5 | 76.0 | 95.0 |
| -3% | 29.1 | 48.5 | 58.2 | 67.9 | 77.6 | 97.0 |
| -1% | 29.7 | 49.5 | 59.4 | 69.3 | 79.2 | 99.0 |
| +1% | 30.3 | 50.5 | 60.6 | 70.7 | 80.8 | 101.0 |
| +2% | 30.6 | 51.0 | 61.2 | 71.4 | 81.6 | 102.0 |
| +3% | 30.9 | 51.5 | 61.8 | 72.1 | 82.4 | 103.0 |
| +4% | 31.2 | 52.0 | 62.4 | 72.8 | 83.2 | 104.0 |
| +5% | 31.5 | 52.5 | 63.0 | 73.5 | 84.0 | 105.0 |
| +7% | 32.1 | 53.5 | 64.2 | 74.9 | 85.6 | 107.0 |
| +10% | 33.0 | 55.0 | 66.0 | 77.0 | 88.0 | 110.0 |
Speedometer Error for Common Tire Swaps
Calculated from the size codes. For any pair use the calculator above or the tire size difference calculator.
| Original | New | Diameter in | Error | True speed at 60 mph | True speed at 100 km/h | 100 indicated miles |
|---|---|---|---|---|---|---|
| 265/70R17 | 285/70R17 | 31.61 to 32.71 | +3.5% | 62.1 mph | 103.5 km/h | 103.5 mi |
| 265/70R17 | 285/75R17 | 31.61 to 33.83 | +7.0% | 64.2 mph | 107.0 km/h | 107.0 mi |
| 225/65R17 | 235/65R17 | 28.52 to 29.03 | +1.8% | 61.1 mph | 101.8 km/h | 101.8 mi |
| 275/65R18 | 295/65R18 | 32.07 to 33.10 | +3.2% | 61.9 mph | 103.2 km/h | 103.2 mi |
| 235/75R15 | 33x12.50R15 | 28.88 to 33.00 | +14.3% | 68.6 mph | 114.3 km/h | 114.3 mi |
| 225/45R17 | 245/40R18 | 24.97 to 25.72 | +3.0% | 61.8 mph | 103.0 km/h | 103.0 mi |
| 205/55R16 | 215/50R17 | 24.88 to 25.46 | +2.4% | 61.4 mph | 102.4 km/h | 102.4 mi |
Speedometer Error Formulas and Sources
Error percentage
(new diameter - original diameter) / original diameter x 100. 265/70R17 to 285/70R17: (32.71 - 31.61) / 31.61 = +3.49%.
True speed
Indicated speed x new diameter / original diameter. 60 x 32.71 / 31.61 = 62.1 mph.
Odometer
Actual distance = indicated distance x new diameter / original diameter. The percentage is the same as the speedometer error.
Direction
A taller tire travels farther per revolution, so the speedometer and odometer read low. A shorter tire makes them read high.
| Source | Statement (paraphrased) |
|---|---|
| Continental Tire, plus-sizing guidance | Match the original rolling circumference closely. A deviation above 3% can affect speedometer accuracy, ABS, stability control and other systems. |
| USTMA, Care and Service of Passenger and Light Truck Tires | Acceptable load capacity or overall diameter does not imply acceptable vehicle clearance. Check clearance at all four wheel positions through full suspension travel and steering lock. |
How to Use the Speedometer Error Calculator
- 1Enter your original (stock) tire size, for example 265/70R17, or choose inch format for flotation sizes.
- 2Enter the new tire size you are considering.
- 3Click Calculate and Compare.
- 4Read the error percentage and the Speedometer tab for true speed at common indicated speeds.
- 5Check clearance and the approved rim width before buying, and ask a dealer about speedometer calibration if the error matters to you.
Frequently Asked Questions
Sources: Continental plus-sizing guidance; USTMA, Care and Service of Passenger and Light Truck Tires. Values are calculated from the size codes. See also the plus sizing options calculator and the winter tire sizing guide.