Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: In the GPS vs car speedometer comparison, the dashboard usually reads a little higher. UN ECE Regulation 39 says a car speedometer must never show less than true speed and limits how much more it may show, so makers build in a margin. GPS at steady speed in open sky is usually closer to true speed, but it.
TL;DR
- Dashboards are allowed to read high, never low.
- A 2–5 mph gap at highway speed is common.
- GPS is best at steady speed under open sky.
- Tyre size and wear shift the car's reading.
- Obey the dashboard if in doubt.
Gps vs car speedometer: the rules behind dashboard error
UN ECE Regulation 39, used in Europe and many other countries, requires that indicated speed is never less than true speed, and that it not exceed true speed by more than 10% plus 4 km/h. In practice that means at a true 100 km/h a car may legally show up to 114 km/h. US federal rules (FMVSS 101) cover the display itself rather than a tolerance; makers still commonly build in a positive margin.
Why GPS often reads lower
Because the dashboard is biased upward, a GPS app often reads a few mph less. GPS velocity from Doppler measurements has no built-in bias, so at steady speed under open sky it tends to sit closer to true speed. See how accurate is a GPS speedometer.
Typical gaps
| Dashboard shows | GPS often shows | Gap |
|---|---|---|
| 30 mph | 28–30 mph | 0–2 mph |
| 60 mph | 56–59 mph | 1–4 mph |
| 70 mph | 66–69 mph | 1–4 mph |
Illustrative ranges only; your car may differ.
When the car is more reliable
In tunnels, multistorey car parks and dense downtown streets the phone may lose or misread satellites. The car's wheel-based speedometer keeps working everywhere. Hard acceleration also exposes GPS lag.
What changes your car's reading
Larger or smaller tyres, worn tread and tyre pressure all change how far the car travels per wheel turn. Use the speedometer error calculator and read why car speedometers read high.
Using both safely
Use Speedometer PRO as a second opinion, set up before you drive and mounted in view. Follow posted limits using whichever reading is higher. Never handle the phone while moving.
Related: GPS speedometer guide · Why car speedometers read high · GPS speedometer accuracy: how accurate is it? · Speedometer error calculator for tyre size changes · How to check speedometer accuracy · Speedometer for car: your phone as a second display · Digital speedometer: big numbers at a glance · Analog speedometer: a classic needle dial
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- UN Regulation No. 39: speedometer and odometer (UNECE): unece.org (accessed 26 October 2026)
- 49 CFR 571.101: controls and displays (eCFR): ecfr.gov (accessed 26 October 2026)
- Speedometer (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- GPS Accuracy (GPS.gov): gps.gov (accessed 26 October 2026)
- Doppler effect (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
FAQ
Why is my GPS speed lower than my speedometer?
Car speedometers are designed to read at or above true speed, so a GPS reading a few mph lower is normal and expected.
Which should I trust for speed limits?
Keep to the limit on your dashboard, which errs on the high side. Use GPS as a check, not as permission to go faster.
Is a 5 mph difference normal?
At motorway speeds a gap of a few mph is common. A bigger, consistent gap may point to a tyre size change or a fault.
Does GPS work better than the car in tunnels?
No. GPS usually loses signal in tunnels, while the car's speedometer keeps working.
Can I recalibrate my car's speedometer?
Dealers or specialists can often adjust it, for example after a tyre size change. Check legality and warranty first.