A GPS Speed Test for Your Car
Your dashboard tells you how fast the wheels are turning. The GPS car speed test on this page tells you how fast the car is actually moving, from your phone's satellite fix, and then compares the two. It is the quickest way to find out how far your speedometer is off, to check it after new wheels or tyres, to keep an eye on an old car whose gauge has started to wander, or to get a second opinion in a hire car.
Running the Test: Steady Speed, Two Readings
A fair comparison needs the car at a constant speed. GPS reports about once a second, so while you accelerate or brake it trails the dashboard for a moment and the two disagree for reasons that have nothing to do with accuracy. Pick a straight, open stretch, settle at a speed above 30 km/h (19 mph) — cruise control makes this easy — and wait a few seconds for both numbers to stop moving.
Then a passenger reads both at the same moment and enters them in the speedometer check, or presses its live GPS button to take the phone's figure directly. Repeat at two or three speeds, for example 50, 80 and 110 km/h. If the gap grows with speed, the error is proportional, which is what a tyre-size effect looks like; if it stays about the same, a fixed offset in the instrument is the likelier cause.
Reading Your Result
- Spot on — the two agree to within one unit, as close as two displays showing whole numbers can be read. Your speedometer is set very close to your true speed.
- Within the normal range — the dashboard reads a little high, inside what UN Regulation No. 39 allows a new car. This is the usual result, and it means a reading of 100 km/h on the dial is a little under 100 in reality.
- Reads higher than allowed — check the tyres first: worn ones read high, and so does a smaller size than the car was built with. Then repeat the test somewhere with a better signal before drawing conclusions.
- Reads low — the one result a new car should never give. If it happens on repeated runs with a good signal, for example after fitting larger wheels, have the speedometer checked: you are going faster than the dashboard says.
Why cars are built to read high at all, and how GPS measures speed, are covered in the guide to how accurate a GPS speedometer is.
How Tyres Change Your Speedometer
A car works out its speed by counting wheel turns and multiplying by how far one turn carries it. That distance is set for the tyre size the car left the factory on. Fit a tyre that is taller overall and each turn carries the car further than the dashboard assumes, so it under-reads; fit a smaller one and it over-reads by the same proportion.
You can work out a tyre's overall diameter from the code on its sidewall: 205/55 R16 means 205 mm wide, a sidewall 55% of that width, on a 16-inch rim. The table compares common sizes with a car set up for 205/55 R16, before the speedometer's own built-in over-read is added on top.
| Tyre | Diameter | Change | At a true 100 km/h |
|---|---|---|---|
| 205/50 R16 | 611 mm | −3.2% | 103.4 km/h |
| 205/55 R16 as built | 632 mm | — | 100 km/h |
| 225/45 R17 | 634 mm | +0.4% | 99.6 km/h |
| 225/40 R18 | 637 mm | +0.8% | 99.2 km/h |
| 215/60 R16 | 664 mm | +5.1% | 95.1 km/h |
Tread wear works the same way on a smaller scale. Every millimetre of tread that wears away comes off both sides of the tyre, so on a 205/55 R16 each millimetre shortens the diameter by about 0.32%. A set that has lost 6 mm of tread makes the dashboard show about 101.9 km/h at a true 100 — and fitting new tyres takes that away again, which is why the reading can drop slightly after a tyre change.
Setting Up HUD Mode on the Windscreen
A head-up display puts your speed in the windscreen so you never look down for it. To do the same with a phone, park, open HUD mode and press Mirror, then lay the phone flat on top of the dashboard with the screen facing up, just below the glass in front of you. The glass reflects the screen, and the mirrored digits read the right way round in the reflection.
It works best at night, with the phone's brightness turned up and nothing bright behind it; in daylight the reflection is faint. Move the phone until the digits sit low in your view rather than over the road ahead, and set it all up before you drive. Some phones dim themselves when they get warm, so on a sunny dashboard the reflection can fade.
Driving Abroad: Units and Local Limits
A hire car in another country can come with a dial in the unit you do not think in, or limits you have not memorised. Switch the speedometer to km/h or mph to match the road signs, and set the speed alert to the local limit, a little under it to allow for GPS error. The dial turns red above it, as a reminder rather than a safety system. To convert a figure on a sign or in a rental agreement, use the speed converter.
When to Trust the Dashboard Instead
GPS is the better reference on an open road at a steady speed. The dashboard is the better one in a tunnel or underground car park, where there is no satellite signal; among tall buildings, where reflections can briefly pull the GPS figure off; and while accelerating or braking hard. Whatever either says, the posted limit is the one that counts, and a phone is never evidence of the speed you were doing.