Speedometer Error Calculator
Calculate the difference between what your speedometer reads and how fast you are actually traveling.
Quick Answer
If you replace factory 31.6-inch tires (265/70R17) with larger 32.7-inch tires (285/70R17), your speedometer will read 3.5% too slow. When your speedometer indicates you are driving 60 mph, your true actual speed on the road is 62.1 mph.
Enter Tire Specifications
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 vs Indicated Speed (265/70R17 → 285/70R17, 3.5% error)
| Speedometer Reading | True Actual Speed | Error Difference |
|---|---|---|
| 30 mph | 31.0 mph | +1.0 mph |
| 40 mph | 41.4 mph | +1.4 mph |
| 50 mph | 51.7 mph | +1.7 mph |
| 60 mph | 62.1 mph | +2.1 mph |
| 70 mph | 72.4 mph | +2.4 mph |
| 80 mph | 82.8 mph | +2.8 mph |
Formula
Actual Speed = (New Tire Diameter / Old Tire Diameter) × Speedometer Reading
- New Tire Diameter = overall height of installed tires in inches
- Old Tire Diameter = overall height of factory OEM tires in inches
- Example: (32.7 / 31.6) × 60 mph = 62.1 mph true speed