Calculating your actual speed is straightforward once you know the Speedometer Error formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Speedometer Error Calculator.
What is Speedometer Error?
The Speedometer Error calculation tells you your actual speed from a few simple inputs. The figure you are solving for here is the actual speed.
The Speedometer Error formula
The core formula is:
Actual speed = Indicated speed × New tyre diameter ÷ Original tyre diameterHere is what each input means:
- Original tyre diameter — a value measured in mm. Example: 632 mm.
- New tyre diameter — a value measured in mm. Example: 650 mm.
- Indicated speed — a value measured in km/h. Example: 100 km/h.
How to calculate it step by step
- Write down the original tyre diameter (for example, 632 mm).
- Write down the new tyre diameter (for example, 650 mm).
- Write down the indicated speed (for example, 100 km/h).
- Apply the formula above to get your actual speed.
- Double-check the result with the Speedometer Error Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Original tyre diameter | 632 mm |
| New tyre diameter | 650 mm |
| Indicated speed | 100 km/h |
| Actual speed | 102.85 |
| Speedometer error | 2.85% |
With original tyre diameter of 632 mm, new tyre diameter of 650 mm and indicated speed of 100 km/h, the actual speed works out to 102.85.
Example 2
With original tyre diameter of 1,300 mm, new tyre diameter of 650 mm and indicated speed of 100 km/h, the actual speed works out to 50.00.
| Result | Value |
|---|---|
| Actual speed | 50.00 |
| Speedometer error | -50.00% |
Example 3
With original tyre diameter of 320 mm, new tyre diameter of 650 mm and indicated speed of 100 km/h, the actual speed works out to 203.13.
| Result | Value |
|---|---|
| Actual speed | 203.13 |
| Speedometer error | 103.13% |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Speedometer Error Calculator does it instantly, for free, with the formula and a worked example built in.
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