Speedometer error calculator for new tires or gears
Enter the original and new tire sizes (and the axle ratios, if those changed) to see how far off the speedometer and odometer are, from 30 to 80 mph.
A ratio change only moves the speedometer when the speed signal is read before the axle (a transmission output sensor). If the vehicle reads speed from the wheel-speed sensors, only the tire change counts.
Tire diameters here are nominal, from the size. For an exact figure, use the maker's published diameter or revs per mile, or measure against a GPS speed.
| Speedometer shows | Actual speed | To drive this fast, it shows |
|---|
With bigger tires, the speedometer reads low by the change in tire diameter. Actual speed = indicated speed × new diameter ÷ old diameter. Going from 265/70R17 (31.6 in) to 285/70R17 (32.7 in), an indicated 60 mph is really about 62.1 mph. If the speed signal comes from the transmission, a new axle ratio changes it too: multiply by old ratio ÷ new ratio.
How to use the Speedometer Error Calculator
Enter both tire sizes
The original and the new size, or each tire's diameter in inches.
Add axle ratios if changed
Only when the ring and pinion changed and the speed signal comes from the transmission.
Read the table
Actual speed at 30, 45, 60, 70 and 80 mph indicated, the reverse, and the odometer error.
What the speedometer error calculator works out
Tire size or diameter
Metric sizes, flotation sizes or a measured diameter in inches.
Axle ratio changes
Old and new ratio, for vehicles that read speed before the axle.
Speed table
Indicated vs actual speed at five speeds, and what it shows when you drive a given speed.
Odometer error
Miles shown vs miles actually driven, by the same factor.
Private
Runs in your browser. Nothing is uploaded.
- Actual speed
- indicated × (new diameter ÷ old diameter)
- With an axle ratio change
- × (old ratio ÷ new ratio), for a transmission-output speed signal
- Odometer
- Off by the same factor as the speedometer
- Diameter
- Nominal, from the size: 2 × width × aspect ÷ 100 ÷ 25.4 + rim
Questions about the speedometer error calculator
Do bigger tires make the speedometer read slow?
How do I calculate my actual speed after a tire change?
Does changing the axle ratio affect the speedometer?
Is the odometer wrong too?
How do I fix the speedometer after a size change?
Where the formulas and rules come from
- Tire Rack: How do I calculate tire dimensions?
- NHTSA: Tire safety, everything rides on it (reading the sidewall)
Checked October 2026. Estimates from nominal tire diameters. Confirm with a GPS speed check after any change.
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