Speedometer Calculator for Bigger Tires
Worked example
Fitting 285/70R17 on a vehicle set for 265/65R17: the diameter goes from 30.6 in to 32.7 in, about 7.0 percent more. When the speedometer says 60 mph, the vehicle is doing about 64 mph, and 100 indicated miles is about 107 real miles.
+7.0% the speedometer reads low: you are going faster than it says
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 21.4 mph | +1.4 mph |
| 30 mph | 32.1 mph | +2.1 mph |
| 45 mph | 48.2 mph | +3.2 mph |
| 60 mph | 64.2 mph | +4.2 mph |
| 70 mph | 74.9 mph | +4.9 mph |
| 80 mph | 85.6 mph | +5.6 mph |
The odometer is off by the same amount: 100 indicated miles is about 107.0 real miles. Check against a GPS speed on a clear stretch before you trust the numbers.
Nominal sizes from the formulas. Real tires differ by maker and tread; the maker's spec sheet lists the actual diameter.
How to use this tool
- Enter the original size
Use the size the vehicle was set up for, usually the one on the door placard. - Enter the new size
Type the size now on the wheels. Metric and flotation both work. - Read the table
Find your usual cruising speed. The Actual speed column is what a GPS should show.
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How it is calculated
- Actual speed = indicated speed x new diameter / old diameter.
Results are nominal. Real tires differ by maker and tread, and the maker's spec sheet lists the actual diameter.
Frequently asked questions
Why does my speedometer read wrong after a tire change?
The speedometer counts wheel turns and assumes a certain tire size. A taller tire covers more ground per turn, so the real speed is higher than shown. A shorter tire does the opposite.
How do I calculate actual speed from indicated speed?
Actual speed = indicated speed x new diameter / old diameter. The tool applies that at six speeds so you do not need to.
Does the odometer have the same error?
Yes. It counts the same wheel turns, so mileage under-counts with a taller tire and over-counts with a shorter one.
How much speedometer error is acceptable?
That is a legal and safety question for you and your local rules, not a number we can set. One tire maker suggests staying within about 3 percent of the original diameter to keep the error small.
How can I check the real speed?
Compare with a GPS speed on a clear, level stretch at a steady speed. The calculator is an estimate from nominal sizes, while GPS measures the real thing.
Can the speedometer be corrected?
Many vehicles allow recalibration through a shop, a programmer or a speedometer calibrator, depending on the model. Check your owner's information or ask a shop.
Link to this tool
Your numbers are saved in the link, so anyone who opens it sees the same result.
Sources
Last checked 2026-11-12. Each link was opened and read on that date.
- BFGoodrich: Changing tire size on a car: A different overall diameter changes speedometer and odometer accuracy.
- BFGoodrich: How to read a tire sidewall: Size code: width in millimeters, aspect ratio, R for radial, rim diameter, P and LT prefixes.
This guide reflects independent editorial research, not first-hand product testing, professional mechanical training, or rescue certification. Some links on this page are Amazon Associates affiliate links; see our affiliate disclosure.