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GPS Speed vs Car Speedometer

If you've ever compared a GPS speed app to your car's speedometer, you've probably noticed they disagree. Here's why — and which one to trust.

You're driving at 100 km/h according to your car. You glance at a GPS speedometer app on your phone and it says 95 km/h. Which one is right?

Short answer: the GPS is almost certainly closer to your actual speed. But the full explanation involves tire physics, international regulations, and the way satellites measure distance.

How your car's speedometer works

Your car measures speed mechanically. A sensor on the transmission or wheel hub counts how many times the wheels rotate per second. The car's computer multiplies rotations by the tire circumference to calculate speed.

This works well in theory, but there's a catch: the tire circumference changes. Tire pressure, tread wear, temperature, and load all affect the actual size of the tire. A tire that's slightly underinflated has a smaller rolling circumference, so the car thinks the wheels are spinning faster than they're actually covering ground.

On top of the physical limitations, car manufacturers deliberately make speedometers read high. This isn't a defect — it's a legal requirement.

Why car speedometers always read high

International regulations (UN ECE Regulation 39) require that a car's speedometer must never underreport speed. The displayed speed must be between the actual speed and 110% of the actual speed plus 4 km/h.

In practice, this means:

At a displayed 50 km/h, your actual speed could be 44-50 km/h
At a displayed 100 km/h, your actual speed could be 87-100 km/h
At a displayed 130 km/h, your actual speed could be 113-130 km/h

Most manufacturers calibrate to read about 3-7% high across the speed range. They err on the side of caution because reading low could expose them to legal liability if a driver gets a speeding ticket while their speedometer showed a legal speed.

The offset isn't always consistent. Tire wear, different tire sizes (if you've replaced your tires with a non-stock size), and even altitude can shift the error. This is why your car might read 5 km/h high at 60 but 8 km/h high at 120.

How GPS speed works

GPS measures speed fundamentally differently from your car. Instead of counting wheel rotations, it tracks your actual position in space using satellite signals.

Your phone receives signals from multiple GPS satellites (typically 8-12 at once). By measuring the time it takes for each signal to arrive, the GPS chip calculates your position. By comparing your position at different moments, it calculates how fast you're actually moving across the ground.

This is called "ground speed" — your actual speed relative to the Earth's surface. It doesn't care about tire size, tire pressure, or wheel rotations. It directly measures how fast your position is changing.

How accurate is GPS speed?

Under good conditions (clear sky, no obstructions), GPS speed on modern iPhones is accurate within 1-3 km/h. Newer iPhones (12 and later) use dual-frequency GPS (L1 + L5 bands), which reduces errors from atmospheric interference and signal reflection.

GPS speed has some limitations:

Tunnels and parking garages — No GPS signal means no speed data. The app will either show zero or the last known speed.
Urban canyons — Tall buildings can reflect GPS signals, causing momentary inaccuracies. Speed might jump a few km/h.
Dense forest — Heavy tree cover can weaken signals, though modern multi-frequency GPS handles this much better.
Rapid acceleration — GPS updates position 1-5 times per second (on phones). During rapid speed changes, there can be a fraction-of-a-second lag. This matters for sprint timing but not for normal driving.

For steady-state driving on open roads, GPS speed is effectively your true speed. The 1-3 km/h uncertainty is significantly smaller than the 3-10 km/h overreading from your car's speedometer.

Which should you trust?

For knowing your actual speed: trust GPS. It measures real ground speed without the deliberate overreading built into car speedometers.

For avoiding speeding tickets: use your car speedometer. Since it reads high, you have a built-in buffer. If your car says 50, you're probably doing 46-48. Speed cameras measure your actual speed, so the speedometer's high reading protects you.

For comparing performance or tracking drives: use GPS. If you want to know your actual top speed, actual average speed, or real acceleration time, GPS gives you the unbiased number. Your car's speedometer would make all of these look slightly better than they actually are.

Fun experiment: check your car's offset

Next time you're on a straight highway at a steady speed, compare your car's speedometer to a GPS speed app. You'll likely see a consistent offset:

Car shows 100 km/h, GPS shows ~94-97 km/h: Normal. Most cars are 3-6% high.
Car shows 100 km/h, GPS shows ~90-93 km/h: Your tires might be underinflated or worn, or your car has a larger factory offset.
Car shows 100 km/h, GPS shows ~98-100 km/h: Unusually accurate car, or you might have slightly larger tires than stock.
Car shows 100 km/h, GPS shows 101+ km/h: Very rare. This would mean your speedometer reads low, which shouldn't happen on a stock car. Check your tire size — oversized aftermarket tires can cause this.

Once you know your car's offset, you can mentally adjust whenever you glance at the speedometer. Or just use a GPS speed app like Rovy and see your real speed directly.

The bottom line

Your car's speedometer is intentionally inaccurate — it reads high by design and by law. GPS speed on your iPhone gives you a much closer picture of how fast you're actually going. Neither is perfect, but for everyday driving, GPS speed is the more truthful number.

If you want to see both side by side while you drive, a GPS speedometer app running on a mounted phone (or via the CarPlay interface) gives you instant access to your real speed whenever you want it.

See your real speed

Rovy's GPS speedometer shows your actual speed. Free on the App Store.

Download on theApp Store