Is my speedometer reading too high?

Almost certainly, and on purpose. Regulations such as UNECE R39 forbid a speedometer from ever reading below true speed and allow a generous margin above it, so manufacturers calibrate high to stay legal across tire wear, pressure and load. A typical car reads 1 to 3 percent above true speed, often showing 70 when you are doing 67 to 69. An error inside roughly 5 percent is normal calibration, not a fault. Motorcycles run higher still, commonly 5 to 10 percent.

How do I calculate speedometer error?

Hold a steady speed, note what the dashboard shows and what GPS shows at the same moment, then compute error = (dashboard − GPS) ÷ GPS × 100. Reading 70 against a GPS 67 gives (70 − 67) ÷ 67 = 4.5 percent high. Your true speed at any dashboard reading is then dashboard ÷ 1.045. The percentage stays roughly constant across speeds because the cause — a stored tire circumference that no longer matches the real one, plus a deliberate calibration offset — scales with speed.

Why does my speedometer differ from GPS?

They measure different things. The dashboard counts wheel rotations and multiplies by a tire circumference stored at the factory, so tread wear, pressure, load and any size change shift the result — and regulation pushes the calibration high on top. GPS measures ground speed from the Doppler shift of satellite signals, independent of the car, typically within 0.5 to 2 mph outdoors. When they disagree at a steady speed, GPS is the honest number.

Can I fix my speedometer error?

If the error came from a tire or wheel size change, yes: the tire size parameter in the control module can be corrected with a dealer tool on many makes, and aftermarket recalibration modules exist for common platforms. If it is just the factory calibration, there is nothing to fix — the bias is intentional and legal. Knowing your percentage and applying it mentally, or glancing at GPS, is the practical answer. What you must never alter is the odometer total, which is illegal.

Does speedometer error change with speed?

As a percentage it stays roughly constant, which is why one measurement gives you a usable correction table. In absolute terms it grows with speed: a 4 percent error is about 1 mph at 25 and about 3 mph at 75. Measure at a steady highway speed where GPS is most stable, and if you want more confidence, repeat at a second speed and compare the percentages — they should land close together.

Tools · Speedometer error calculator

Speedometer error calculator: dashboard vs GPS.

Enter the two readings from your drive and get your personal error percentage, plus a table of what your dashboard really means at every speed. Works in mph or km/h — the math is the same.

Dashboard shows GPS shows

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Dashboard	Estimated actual

Getting your two numbers

Hold a steady speed on a flat, straight stretch — cruise control is ideal — and read the dashboard and a GPS speedometer at the same moment. The car speed test walks through the full 10-minute procedure with the live GPS tool on the page; a passenger reading the phone is the safe way to do it. Repeat at a second speed if you want confidence: the two percentages should land close together.

What counts as normal

Error	Verdict
0–2% high	Tight calibration, nothing to do
2–5% high	Typical factory calibration, working as designed
5–10% high	Normal on motorcycles; on a car, check tire size and wear
10%+ high	Usually a tire/wheel change the car was never told about
Reads low	Unusual — recheck the measurement; if real, larger tires are fitted

The bias is deliberate. Rules like UNECE R39 forbid a speedometer from ever under-reading and allow a margin above true speed, so manufacturers aim high to stay legal across tread wear, pressure and load. We unpack the regulation and the mechanics in why your speedometer disagrees with GPS.

If the error came from your tires

A size change moves the rolling circumference the car multiplies by, and the dashboard drifts by the same ratio at every speed. The tire size calculator predicts that shift from the tire specs — useful before you buy — while this page measures the result after the fact. If the two disagree, believe the measurement. Fixes and their costs are covered in the recalibration guide.

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