You're cruising on the highway. Your dashboard reads 60 mph. You glance at a GPS speed app on your phone and it says 57. Neither gauge is broken — they're measuring speed in fundamentally different ways, and one of them is designed to be a little wrong.
Car speedometers are built to over-read, on purpose
This isn't a manufacturing defect — it's policy. In the EU and UK, vehicle regulations require speedometers to never show a speed lower than your actual speed, but they're allowed to show a speed that's higher by a defined margin (historically up to about 10% plus 4 km/h). Manufacturers calibrate to that allowance because the legal and safety risk of a speedometer under-reading is far worse than one that over-reads slightly. The U.S. doesn't mandate an identical formula, but manufacturers follow the same logic for the same reason: nobody wants to get sued because a customer's speedometer said they were under the limit when they weren't.
The practical effect: a brand-new car straight from the factory will, on average, show you a number that's a few percent higher than your true ground speed — often in the 2–5% range at highway speeds.
Tires change the number over time, too
A speedometer doesn't measure speed directly — it counts wheel rotations and multiplies by the tire's assumed rolling circumference. That assumption gets baked in at the factory for a specific tire size. Two things quietly break it over time:
- Tire wear. A worn tire has a slightly smaller effective radius than a new one, which means more rotations per mile and a speedometer that reads a bit high relative to a fresh set.
- Wrong replacement size. Fit a slightly smaller wheel/tire combo than spec (common with some aftermarket or "plus-sized" wheel changes) and the error can grow well beyond the factory tolerance — in either direction depending on which way the size changed.
Neither of these will trigger a warning light. The only way most drivers ever notice is by comparing against an independent source — which is exactly where GPS comes in.
So how does GPS arrive at its number?
GPS doesn't have a speedometer either, in the literal sense — there's no rotating part to count. Your phone or GPS chip works your speed out from satellite signals, usually one of two ways: directly, from the Doppler shift of the signals as you move, or indirectly, by measuring how far you traveled between two GPS fixes and dividing by the time elapsed. We cover exactly how that math works in a separate plain-English breakdown — the short version is that it's a physics-and-geometry calculation, not a mechanical one, so it isn't biased high or low the way a speedometer is. It has its own sources of error instead — fewer visible satellites, signal bouncing off buildings, a phone GPS chip that's cheaper and less precise than a dedicated automotive unit.
Which one should you actually trust?
For day-to-day driving, trust your speedometer for staying within the speed limit — it's calibrated to never show you going slower than you really are, which is the safer direction to be wrong in, and it updates instantly with no satellite lag. Use GPS speed as a sanity check, not a replacement: it's genuinely useful for noticing a speedometer that's drifted out of its normal range (a strong sign of tire wear or a wheel/tire size change worth investigating), and it's the more honest, unbiased number when you actually need to know your true ground speed — for example, pacing yourself against a known distance, or comparing speed across different vehicles.
What GPS speed is not a good substitute for is a legal defense against a speeding ticket — courts rely on calibrated, certified instruments (radar, lidar, or the vehicle's own speedometer), not a consumer phone app, and for good reason: a phone's GPS accuracy can vary by the second depending on sky visibility and satellite geometry.
Quick takeaway
If your speedometer and GPS disagree by a small, consistent amount — a few percent — that's expected and normal, not a fault. If the gap is large, growing, or unusually inconsistent, it's worth checking your tire size and tread wear. Either way, our car speed checker gives you a free, no-install way to run that comparison for yourself, right in your browser.