Speedometer Error Calculator
Check how a tire size or gear change affects your speedometer accuracy and actual speed with this free speedometer error calculator online.
Your tyres
Enter a tyre size (265/70R17) or a diameter in mm for each tyre. Runs in your browser.
Result
| Speedo reads (km/h) | Actual (km/h) |
|---|---|
| 40 | 41.4 |
| 60 | 62.1 |
| 80 | 82.8 |
| 100 | 103.5 |
| 110 | 113.8 |
Bigger tyres: you travel faster than the speedo shows and the odometer reads low.
Estimates only — check before you fit
These results are calculated from tyre-size geometry and the values you enter. Real tyre dimensions vary by brand, model, load, pressure and wear. Always check the vehicle placard, tyre manufacturer data, local road rules, and guard / brake / suspension clearance, and get professional fitment advice before changing tyre or wheel sizes. This tool does not judge roadworthiness or legality.
How to use this calculator
- Enter your original (OEM) and new tyres — either a full size like 265/70R17 or a diameter in mm.
- Set the indicated (speedo) speed and choose km/h or mph.
- Read the actual speed, the speedometer error %, the fitment-risk badge and the speed table.
How it works
A tyre's road speed is proportional to its rolling circumference, which is proportional to its diameter. So actual speed = indicated speed × (new diameter ÷ original diameter), and the speedometer error is the same ratio expressed as a percentage.
Bigger tyres make you travel faster than the speedo shows (and the odometer reads low); smaller tyres do the opposite. The risk badge bands the diameter change: under 1% low, 1–3% check, over 3% high.
Worked example
632 mm → 650 mm at 100 km/h. Actual speed ≈ 102.85 km/h, about +2.85% — your speedo reads low by that amount.
Frequently asked questions
Can I enter a tyre size instead of a diameter?
Yes. Type a size like 265/70R17 or 33x12.50R15 and the tool works out the diameter for you, or enter the diameter in mm directly.
Will bigger tyres fail inspection?
Many regions limit how much overall diameter can change. This tool only estimates the speedo effect — it does not judge roadworthiness. Check your local road rules.
Does this fix my speedometer?
No — it shows the error. Correcting a speedo usually needs a recalibration device or a workshop. Use the figures here as a guide only.
Are these results safe to fit?
They're geometry estimates only. Real tyres vary by brand, load, pressure and wear. Always check the vehicle placard, manufacturer data, local road rules and guard/brake/suspension clearance, and get professional fitment advice before changing tyre or wheel sizes.
Related tools
- Tire Size Calculator
- Tire Size Comparison Visualizer
- Effective Gear Ratio Calculator
- 4WD Tyre Clearance Change Calculator
- Tire Pressure Change Assistant
- Alternate Tire Size Finder
Explore more in Automotive, 4WD, Towing & EV.
Cite this tool
Zerdly. (2026). Speedometer Error Calculator. Retrieved from https://www.zerdly.com/tools/speedometer-error-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.