GPS speed vs. car speedometer: which is right?
If your car says 100 and your phone says 96, your phone is probably the more accurate one — on purpose.
Car speedometers are built to read slightly high
In many regions (including EU type-approval rules), automotive speedometers are required to never under-read your true speed, and are permitted to over-read by a margin. Manufacturers use that margin deliberately, so a car speedometer showing 100 km/h typically means your true ground speed is a bit lower — often 2–5% less. This is a legal safety buffer, not a defect.
Tire size and pressure shift the error further
Car speedometers infer speed from wheel rotation, calibrated against a factory tire size and pressure. Under-inflated tires, worn tread, or a different tire size than the factory spec (including many aftermarket wheel/tire combinations) will change the effective rolling diameter and shift the speedometer’s error — usually making it read even higher than actual speed.
GPS measures your actual ground speed
GPS speed isn’t derived from your vehicle at all — it’s your device’s true rate of movement over the ground, calculated from satellite signals. It has its own error margin (see our GPS accuracy breakdown), but that error isn’t systematically biased high or low the way a car speedometer’s is.
So which should you trust?
For everyday driving, your car’s speedometer is what you’re legally expected to obey, and it’s designed to never show a speed below your true one. If you’re curious about your actual ground speed — for cycling, running, boating, or general curiosity — GPS speed from a phone is generally the closer estimate, provided you have a clear GPS signal.
Check your current speed on our online speedometer, or read how accurate GPS speed really is.