Why Your GPS Speed Is Lower Than Your Car's Speedometer

Almost every driver who installs a GPS speedometer asks the same question within the first ten minutes: why is the phone showing less than the dashboard? The answer is not that one of them is broken. The gap is designed in, and its size is written into regulation.

The rule behind the gap

In Europe and in every market that follows UNECE Regulation 39, a vehicle's speedometer must satisfy two conditions:

• The indicated speed must never be lower than the true speed.
• The indicated speed must not exceed the true speed by more than 10 percent plus 4 km/h.

Read that carefully. Under-reading is forbidden outright; over-reading is allowed within a generous band. A manufacturer who calibrated for perfect accuracy would risk falling on the wrong side of the rule as tyres wear, so nobody does. They bias the reading upward and stay comfortably legal.

At an indicated 100 km/h, that allowance means your true speed could legitimately be anywhere from about 87 to 100 km/h. Most production cars land 3 to 7 percent high.

Why tyres matter more than you would think

A car derives speed from wheel rotation multiplied by the rolling circumference it was calibrated against. Anything that changes the real circumference shifts the reading:

• Wear. A tyre worn from 8 mm to 2 mm of tread has a rolling radius about 6 mm smaller, which alone adds roughly half a percent to the indicated speed.
• Pressure. Under-inflation slightly reduces the rolling radius and nudges the reading up.
• Different tyre size. Fitting a size the car was not calibrated for can shift the reading by several percent in either direction, and this one can push the speedometer into under-reading, which is the illegal direction.

GPS is immune to all of it. It never touches the wheels.

When the difference means something

A steady few percent is normal and uninteresting. What deserves attention:

• The gap suddenly changes after new tyres or a wheel change — the new size probably is not what the car expects.
• The phone reads higher than the car. This is the direction regulation forbids and it is worth investigating, once you have ruled out a poor GPS fix.
• The gap is wildly inconsistent minute to minute. That is almost always the GPS, not the car: check whether you are driving between tall buildings or under heavy tree cover.

Practical advice

For staying legal, use the dashboard. It is biased in the safe direction: if it says 50, you are doing 50 or less. Use the GPS reading for the things the dashboard cannot tell you — how fast you actually covered a stretch of road, your genuine average over a trip, whether a stretch of motorway was as slow as it felt.

In Speedometer both live together: the current GPS speed on the face you prefer, and behind it a trip log with distance, average speed, maximum speed and the route on a map. Set your own limit in settings and the app alerts you when you pass it, so you can keep your eyes up rather than on either number.