Speedometer Error Calculator
Calculate speedometer error from a tire size change.
Enter original and new tire dimensions to find your true speed and how much the reading is off by.
Your speedometer calculates speed by counting wheel rotations per unit time and multiplying by the tire’s circumference. Change the tire size and the circumference changes — but the speedometer still uses the original circumference, so it reads incorrectly.
Tire outer diameter (mm) = (wheel diameter in inches × 25.4) + (2 × section width × aspect ratio / 100) Tire circumference = π × outer diameter
The error: True speed = Indicated speed × (New circumference / Original circumference) Error % = (New circumference / Original circumference - 1) × 100
If the new tire is larger than the original, each wheel rotation covers more ground. The speedometer still counts the same rotations but underestimates how far you have traveled. Your true speed is higher than indicated — potentially enough to affect speeding tickets.
Example: switching from 215/65R15 to 235/65R15:
- Original OD: (15 × 25.4) + (2 × 215 × 0.65) = 381 + 279.5 = 660.5 mm
- New OD: (15 × 25.4) + (2 × 235 × 0.65) = 381 + 305.5 = 686.5 mm
- At an indicated 60 mph, true speed = 60 × (686.5 / 660.5) = 62.4 mph (+4%)
Manufacturers calibrate speedometers to read slightly high for safety and legal reasons. Most OEM speedometers read 2-4% above true speed at the original tire size. A tire change that makes the speedometer read 5% high on top of this baseline could put you 3-4 mph fast on a 70 mph freeway.
Modern vehicles with TPMS (tire pressure monitoring) may require recalibration after a significant tire size change.