Why GPS Speed and Your Car Speedometer Differ

A difference between your dashboard and GPS speed does not by itself prove either instrument is broken. They use different inputs and may update at different times.

Two ways to estimate speed

A car commonly derives speed from wheel or drivetrain rotation and a calibrated relationship to distance. Tire size, wear and pressure can influence that relationship. Your phone receives a speed estimate from its location provider; satellite reception, filtering and update intervals affect what you see.

Some vehicle approval standards, including UN Regulation 39, constrain speedometer error under specified test conditions. That does not establish a universal offset for every car or country. Avoid assuming that subtracting a fixed number from the dashboard always gives true speed.

Make a useful comparison

• Confirm both displays use the same unit: 60 mph is about 96.6 km/h.
• Compare steady motion on an open stretch, rather than a moment of acceleration or braking.
• Let a passenger observe the phone, or review available data after stopping.
• Repeat under similar conditions; one snapshot is weak evidence.

For example, a dashboard showing 100 km/h while a phone shows 96 km/h is an observation, not a calibration certificate. Poor GPS reception can also create discrepancies in the other direction.

When to investigate

If the difference changes suddenly after tire replacement or vehicle work, check the approved tire specification and ask a qualified technician. If it only appears beside tall buildings or in tunnels, inspect location reception first.

Which reading should I use?

Use the vehicle’s instruments for driving and obey posted limits. SpeedX is useful for understanding and comparing readings, but a phone display does not justify driving faster or certify a vehicle’s speedometer.

Try it and keep reading

• Online speedometer
• Fix missing GPS speed readings

References

• UNECE: UN Regulation 39