Which is more accurate, GPS or my car's speedometer?

Under good GPS conditions (open sky, good signal), GPS speed is usually closer to your true ground speed. Vehicle speedometers are mechanically simple and reliable but are affected by tire size changes and are commonly calibrated to never read slower than actual speed, which biases them to read a little high rather than low.

Why does the gap change at different speeds?

A speedometer driven by wheel rotation has an error that scales roughly with speed — if it reads 3% high, that gap grows in absolute terms as speed increases, whereas GPS error is mostly independent of how fast you're going.

Does tire pressure really affect it?

Yes. Underinflated tires have a smaller rolling radius, so the wheel (and the sensor reading its rotation) turns more times per distance traveled, which skews a wheel-speed-based speedometer even if nothing else about the car has changed.

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GPS Speed vs. Your Car's Speedometer

Why your car's speedometer and a GPS speed reading rarely match exactly, and which one is closer to your true speed.

Two different ways of measuring speed

A car's factory speedometer typically derives speed from how fast a wheel (or the output shaft) is rotating, combined with a known or assumed tire circumference. GPS, by contrast, measures your actual movement over the ground using satellite signals and has no concept of tires, gearing, or rotation at all. The two systems can agree closely, but they're measuring fundamentally different things and are affected by different sources of error.

Why factory speedometers tend to read high

In many regions, regulations permit a vehicle speedometer to read higher than true speed but not lower, as a safety margin against unintentional speeding. Manufacturers commonly calibrate toward this allowance, so it's common for a car's dashboard to show a few percent more than your actual ground speed, especially at highway speeds. This is a general pattern, not a guaranteed number for any specific vehicle or jurisdiction.

On top of that baseline calibration, tire wear, tire pressure, and non-stock wheel or tire sizes all change the effective rolling circumference, which shifts the reading further — usually still in the direction of reading a bit high as tires wear down and their radius shrinks slightly.

Where GPS can be the weaker link

GPS isn't immune to error either. In tunnels, parking structures, and dense urban canyons between tall buildings, GPS accuracy can degrade sharply or drop out altogether, while a wheel-speed-based speedometer keeps working regardless of overhead obstruction. For that reason, neither source should be treated as infallible — see the GPS speed accuracy guide for what affects GPS specifically.

In short: expect small, usually consistent differences between the two, and treat a GPS reading as a useful independent reference rather than a verdict on which number is "right."

Frequently asked questions

Which is more accurate, GPS or my car's speedometer?: Under good GPS conditions (open sky, good signal), GPS speed is usually closer to your true ground speed. Vehicle speedometers are mechanically simple and reliable but are affected by tire size changes and are commonly calibrated to never read slower than actual speed, which biases them to read a little high rather than low.
Why does the gap change at different speeds?: A speedometer driven by wheel rotation has an error that scales roughly with speed — if it reads 3% high, that gap grows in absolute terms as speed increases, whereas GPS error is mostly independent of how fast you're going.
Does tire pressure really affect it?: Yes. Underinflated tires have a smaller rolling radius, so the wheel (and the sensor reading its rotation) turns more times per distance traveled, which skews a wheel-speed-based speedometer even if nothing else about the car has changed.