Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: This speedometer error calculator compares the overall diameter of your original and new tyres. Car speedometers count wheel turns, so a larger tyre makes the speedometer read low and a smaller one makes it read high, by the same percentage as the diameter change. Enter both sizes, such as 205/55R16, and see the error at common speeds.
TL;DR
- Error % = (new diameter ÷ old diameter − 1).
- Bigger tyres → speedometer reads low.
- Smaller tyres → speedometer reads high.
- Odometer error is the same percentage.
- Check against GPS on a straight road.
How tyre sizes encode diameter
A size like 205/55R16 means a tread width of 205 mm, a sidewall height of 55% of the width, and a 16-inch wheel. Overall diameter = wheel diameter in mm + 2 × sidewall height. For 205/55R16: 16 × 25.4 = 406.4 mm, sidewall 112.75 mm, so about 631.9 mm.
Speedometer error calculator method
The car's computer assumes a certain number of wheel revolutions per mile. If the new tyre is 3% larger in diameter, each turn covers 3% more road, so at an indicated 60 mph you are really doing about 61.8 mph. The calculator shows true speed for indicated speeds of 20–80 mph and the odometer error per 1,000 miles.
Example
| Change | Diameter change | Indicated 60 mph is really |
|---|---|---|
| 205/55R16 → 225/45R17 | +0.4% | ≈ 60.2 mph |
| 205/55R16 → 215/60R16 | +5.1% | ≈ 63.1 mph |
| 225/45R17 → 205/45R17 | −2.8% | ≈ 58.3 mph |
Rounded; enter your exact sizes in the tool.
Why it matters
Most car speedometers are built to read slightly high, so a modest increase in tyre size may cancel that margin. A larger change can leave the speedometer reading below true speed, which risks tickets and may conflict with vehicle rules. Tyre wear also shrinks diameter slightly over the tyre's life. Background: tire size speedometer error.
Interpreting the result
Over-reading of a few percent is normal and legal in many regions. Under-reading needs attention.
Check with GPS
After changing tyres, drive a straight, open road at a steady indicated speed and compare with Speedometer PRO or the online speedometer. A consistent gap of several percent suggests recalibration by a dealer or tuner.
Related: GPS speedometer guide · Tire size speedometer error explained · Why car speedometers read high · How to check speedometer accuracy · Free speed tools · Online speedometer: your speed in the browser · Digital speedometer: big numbers at a glance
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- Tire code (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Speedometer (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- UN Regulation No. 39: speedometer and odometer (UNECE): unece.org (accessed 26 October 2026)
FAQ
Will bigger tyres make my speedometer read low?
Yes. A larger overall diameter covers more ground per turn, so the speedometer shows less than your true speed by the same percentage.
How much difference is acceptable?
Many fitters aim to keep overall diameter within a few percent of the original. Check your vehicle maker's guidance and local rules before changing sizes.
Does tyre wear change speedometer readings?
Slightly. A worn tread reduces diameter by a few millimetres, which makes the speedometer read a touch higher than with new tyres.
Is the odometer affected too?
Yes, by the same percentage. Larger tyres make the odometer record fewer miles than you actually drove.
How do I fix the error?
A dealer or specialist can often recalibrate the vehicle's settings for the new size. Otherwise return to the original size.