Why is my GPS speed different from my car speedometer?

The devices use different inputs and update timing. Units and reception can also differ. A comparison alone does not identify the correct reading.

Should I subtract a fixed percentage from my dashboard?

This guide does not establish a correction for your vehicle. A fixed rule would need evidence about the actual vehicle and measurement conditions.

What is the difference between average and top speed?

Average speed is total distance divided by total elapsed time. Top speed is the highest reading. A maximum can be affected by a GPS spike.

Open Road

Car Speedometer vs GPS Speed: Why the Numbers Differ

A car dashboard and a phone GPS speedometer measure speed in different ways. Differences can come from the measurement method, update timing, units or GPS reception. Comparing them does not by itself tell you which reading is correct.

Free to download. Open Road Pro is optional, with a 3-day trial on the yearly plan.

Open Road saved-drive speed graph and speed distribution — A real saved-drive analysis screen. The example readings are not an accuracy benchmark.

Check units before interpreting a difference

A reading in miles per hour cannot be compared directly with one in kilometres per hour. One mph equals 1.609344 km/h. Set both displays to the same unit, or use the converter linked below.

Make sure you are comparing current speed with current speed. A maximum from earlier in the drive and an average over the journey are different measurements, even if they use the same unit.

Different methods and different timing

A vehicle typically derives speed from vehicle sensors. A phone uses location information. During acceleration or braking, screens that refresh at different times can disagree even before considering measurement error.

A route through a tunnel or beside tall buildings can also interrupt phone reception. Wait for stable readings after the signal returns. Do not assume that a GPS number overrides the dashboard or that a constant percentage correction applies to every vehicle.

Use recorded context after parking

Open Road places the route and speed analysis together for a recorded drive. Use that context to distinguish a sustained pattern from an isolated spike. Set up the phone before driving and leave comparisons to a passenger or a later review.

About this guide

Published by Open Road. Updated 2026-11-03. App screens show example recorded drives.

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Questions, answered

Why is my GPS speed different from my car speedometer?: The devices use different inputs and update timing. Units and reception can also differ. A comparison alone does not identify the correct reading.
Should I subtract a fixed percentage from my dashboard?: This guide does not establish a correction for your vehicle. A fixed rule would need evidence about the actual vehicle and measurement conditions.
What is the difference between average and top speed?: Average speed is total distance divided by total elapsed time. Top speed is the highest reading. A maximum can be affected by a GPS spike.

Keep your next drive

Get Open Road for iPhone or Android. Free to download, with optional Pro. The yearly Pro plan has a 3-day trial.