Speedometer Error from Tyre Size
How much a change of tyre size shifts the indicated speed and the odometer reading.
Results
Actual speed
100.38
km/h
Percentage change
0.380
%
Diameter
631.9
mm
Diameter
634.3
mm
What this tool does
A speedometer counts wheel revolutions, so a taller tyre travels further per turn and the dial under-reads. Keeping the overall diameter within about 3 % of the original is the usual rule; beyond that the odometer, the cruise control and any speed limiter all drift with it.
Formula
v real = v stated × new perimeter ÷ perimeter original
Variables
| Symbol | Meaning | Unit |
|---|---|---|
w1 | Tyre width | mm |
a1 | Aspect ratio | % |
r1 | Rim diameter | in |
w2 | Tyre width | mm |
a2 | Aspect ratio | % |
r2 | Rim diameter | in |
s | Speed on the dial | km/h |
V | Actual speed | km/h |
E | Percentage change | % |
D1 | Diameter | mm |
D2 | Diameter | mm |
Worked example
- Tyre width205 mm
- Aspect ratio55 %
- Rim diameter16 in
- Tyre width225 mm
- Aspect ratio45 %
- Rim diameter17 in
- Speed on the dial100 km/h
- Actual speed100.38 km/h
- Percentage change0.380 %
- Diameter631.9 mm
- Diameter634.3 mm
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.