Car speed test

Which speeds to test

Speedometer error is usually a percentage, so the gap in mph or km/h grows as you go faster. Test at a few common speeds — for example 30, 50 and 70 mph, or 50, 80, 100 and 120 km/h. The tables show the highest reading UN regulation R39 allows at each true speed.

Need to translate a reading? 70 mph is 112.65 km/h and 100 km/h is 62.14 mph. For any other value use the mph to km/h or km/h to mph converter.

Why your speedometer reads higher than GPS

It’s by design. UN regulation R39, which applies in the EU, the UK and many other countries, says a speedometer must never show less than the vehicle’s true speed. It may show more — up to 10% of the true speed plus 4 km/h. Because tire wear, tire pressure and production tolerances all shift the reading slightly, manufacturers set speedometers to read a little high so they never under-read.

In the US there is no equivalent federal accuracy requirement, but carmakers follow their own and industry standards, and most American cars also read slightly high. In practice, a difference of a few percent between your dashboard and GPS — often around 2–5% — is normal.

What causes speedometer error

Your car calculates speed from how fast the wheels or the gearbox output shaft turn. Anything that changes the distance covered per turn changes the reading:

• Tire size. Larger-diameter tires cover more ground per turn, so the speedometer reads low. Smaller tires make it read high. A 3% bigger tire turns an indicated 60 mph into about 62 mph.
• Tire wear. A worn tire is slightly smaller than a new one — typically a 1–2% difference in diameter — so the reading creeps up a little.
• Aftermarket wheels and lift kits. Bigger wheels and off-road tires are the most common reason a speedometer suddenly reads low.
• Gearing changes. A different final-drive (axle) ratio or a transmission swap changes the relationship between sensor and wheel speed.
• Calibration and sensors. Factory calibration, a failing speed sensor or wrong settings after a module replacement can all shift the reading.
• Tire pressure. Under-inflated tires are slightly smaller when rolling. The effect is small but adds to the others.

Your speedometer is off — what now?

It reads a few percent high

That’s normal and nothing to fix. Just remember that your real speed is a bit lower than the dashboard shows.

It reads low

This is the case to take seriously: you are going faster than you think, which can lead to speeding tickets. Check that your tires are the size listed on the door-jamb sticker. If you’ve fitted larger wheels or tires, the speedometer can usually be recalibrated by a dealer, a tire or 4×4 specialist, or with a programmer for your vehicle.

It jumps, drops to zero or reads wildly wrong

A faulty vehicle speed sensor, wiring or ABS sensor may be the cause. Have it checked by a mechanic. Until then, our GPS speedometer is a good stand-in.

Getting an accurate GPS reading

GPS is a great reference, but only when the phone has a good signal. For the best results:

• wait for the accuracy badge to show ±25 m or better before comparing;
• keep the phone near the windshield — some heated or metallized windshields weaken GPS;
• avoid tunnels, multi-level interchanges and tall city streets;
• compare only at a constant speed, not while accelerating or braking.

The speedometer also records max and average speed and the distance driven, so you can compare your car’s trip meter with GPS over a longer drive. To work out average speed from a distance and time, try the average speed calculator.