How do I check my speedometer with GPS?

On a straight, open road with a good GPS signal, hold a steady indicated speed (say 60 on the dial) and read the GPS speed. Repeat at a couple of speeds. The consistent gap you see is your car's calibration margin.

What changes a car's speedometer accuracy over time?

Tire diameter: worn tread, non-stock tire sizes, and even pressure changes alter how far each wheel revolution travels, shifting the reading. GPS doesn't care about your tires — which is what makes it the reference.

Is Your Car's Speedometer Accurate?

Q: Why does my car's speedometer read high?

Regulations in most markets forbid speedometers from ever reading below true speed, so manufacturers build in a safety margin — typically a few percent plus a small constant. A dial showing 70 usually means a true 67–68.

Almost certainly not — and that's intentional. Your dashboard has been telling you a small, legally mandated white lie since the day the car left the factory. Ten minutes with GPS tells you exactly how big it is.

The polite lie, explained

Regulators in most markets (the EU's rules are the famous example) require that a speedometer never read below true speed, while allowing it to read somewhat above. Faced with that asymmetric rule plus manufacturing tolerances, every manufacturer calibrates high on purpose — commonly a few percent plus a constant. The result: an indicated 70 is typically a true 67–68. Your car isn't broken; it's compliant.

What moves the number further

Tire wear: the dial reads wheel revolutions. Worn tread shrinks the tire's circumference, so each revolution covers less road — and the dial reads even higher.
Non-stock tires: a different diameter than factory spec shifts the whole calibration, in either direction.
Pressure: underinflation slightly shrinks effective diameter. Small, but it stacks.

GPS ignores all of it — satellites don't know what tires you run — which is precisely why it's the reference measurement.

The ten-minute check

Conditions: straight, open road; light traffic; the app showing a good GPS signal badge. A passenger reads the phone — or HUD mode puts both numbers in your sightline at night.
Hold an indicated 50 (dial, cruise control if you have it) and note the GPS reading.
Repeat at 60 and 70. The offset is usually consistent in percentage terms — that's your car's margin.
Do the math once, remember it forever: “my dial reads 4% high” recalibrates every glance you'll ever make at it.

What to do with the answer

Mostly: relax. Knowing the margin means knowing your true speed on every drive — useful for keeping honest pace with speed limits, judging trip times, and settling the eternal “this car feels faster” debate with data. If the gap is far outside a few percent, check tire size and wear first; that's almost always the culprit.

Rental-car ritual: unknown car, unknown tires, unknown margin — two minutes of GPS comparison on the first highway stretch tells you how much to trust the unfamiliar dial for the rest of the trip.

Audit your dashboard

GPS truth next to the dial's optimism. Free on iPhone.

Download on the App Store

FAQ

Why does my car's speedometer read high?

Regulations forbid under-reading, so manufacturers calibrate a few percent high on purpose.

How do I check it with GPS?

Steady indicated speeds on an open road, good signal, note the gaps — the offset is your margin.

What changes accuracy over time?

Tire diameter — wear, size, pressure. GPS doesn't care, which is why it's the reference.