GPS vs Car Speedometer: Why Your Car Reads Faster Than You're Going
Updated October 19, 2026 · 5 min read
Open a GPS speedometer on your phone while driving and you’ll probably notice it shows a slightly lower number than your car. Your car reads 70 mph; your phone says 67. That isn’t a fault — it’s mostly by design.
The short answer
Car speedometers are built to read at or aboveyour true speed, never below it. A GPS speedometer measures your actual movement over the ground, so in good conditions it’s the more accurate of the two.
Why car speedometers read high
1. Regulations forbid reading low
In the European Union, the UK, and many other countries, cars must meet UN ECE Regulation 39. It says a speedometer may never show less than the true speed, and may show more by up to 10% plus 4 km/h. At a true 100 km/h, a speedometer can legally display anything from 100 to 114 km/h.
US federal rules don’t set a specific accuracy tolerance, but because cars are designed for global markets, manufacturers generally calibrate them the same way: reading slightly high is safe; reading low could get drivers tickets.
2. Tire size and wear
A car speedometer counts wheel rotations and assumes each one covers a fixed distance. Anything that changes the tire’s rolling circumference changes the reading:
- Worn tires are smaller, so each turn covers less ground — the speedometer reads slightly higher.
- Larger or aftermarket wheels and tires cover more ground per turn — the speedometer can read lower than true speed, which is the risky direction.
- Low tire pressure slightly shrinks the effective radius.
3. Gauge design
Analog needles have limited resolution, and some manufacturers add a few percent of deliberate margin on top of the tolerance.
How big is the difference?
It varies by car, but a reading 2–5% high is common. Here’s what that looks like:
| Car shows | True speed if 3% high | True speed if 5% high |
|---|---|---|
| 30 mph | 29.1 mph | 28.6 mph |
| 50 mph | 48.5 mph | 47.6 mph |
| 70 mph | 68.0 mph | 66.7 mph |
| 80 mph | 77.7 mph | 76.2 mph |
So which one should you trust?
- In open sky at a steady speed: GPS is usually closer to your true speed.
- In tunnels, garages, and dense city centers: trust the car. GPS can lose signal or jump.
- For staying under the limit: following your car’s speedometer keeps you on the safe side, because it reads high.
- After changing wheel or tire size: compare with GPS — your car may now be reading low.
How to check your own car’s speedometer
- Mount your phone securely with a view of the sky — or ask a passenger to hold it.
- Open the free online speedometer or a GPS speedometer app and wait for a good GPS fix.
- On a straight, open road, hold a steady speed using cruise control (for example 30, 50, and 70 mph).
- Note both readings at each speed. The percentage difference is your car’s error.
Never look at or touch your phone while driving — set everything up first, or let a passenger read it.
Why GPS can be wrong too
GPS isn’t perfect. It needs a clear view of the sky, updates about once per second, and can lag during hard acceleration. Read more in how a GPS speedometer works.
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