Speedometer Error Calculator
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Larger tires make the speedometer read slow — see actual vs indicated speed.
Tire diameters
Actual speed
67.5
Error: 3.8% (reads slow)
- Diameter ratio
- 1.0385
Related calculators
Speedometer and tire diameter
Your speedometer counts driveshaft or wheel rotations and converts them to speed using the factory tire diameter. Upsizing wheels or tires increases overall diameter — each rotation covers more ground, so the speedometer reads lower than your true speed.
Actual speed ≈ indicated speed × (new diameter ÷ stock diameter). A +3% taller tire means the needle shows 60 mph when you are really doing about 61.8 mph. The reverse applies for smaller tires.
Example: stock 26.0 in overall diameter, new 27.0 in (+3.8%). At an indicated 65 mph, actual speed ≈ 67.5 mph. Pair with our tire size calculator to get diameters from metric tire codes.
Common questions
How much diameter change is OK?
Many owners stay within 2–3% of stock to keep speedometer and ABS calibration acceptable. Check fitment and local rules.
Does this fix my speedometer?
No — it estimates error. Some vehicles can be recalibrated with dealer tools or aftermarket programmers.