What is a car speedometer app?
Mount your phone, press Start, and you get your real ground speed next to your car's own dial. Which is almost certainly reading high. There's a speed-limit alert, a 0 to 100 timer and a record of the drive, all worked out on your phone.
The dial is scaled for the road, so it reads to 240 km/h like a real car. Switch to mph and the whole face rescales with it.
Why is my dashboard different?
This is the question the page exists for, and the answer isn't that one of them is broken.
UNECE Regulation 39 covers speedometers across the EU, the UK and most other markets, and it's deliberately lopsided. Your speedo can never show less than your real speed, but it's allowed to show up to 10% plus 4 km/h more. Nothing says it has to be accurate. Only that it errs upwards.
So carmakers use that room. A car reading low is a legal problem, a car reading high isn't. They set the needle a few percent optimistic and leave space for tyre wear, which shrinks the rolling radius over the years and would otherwise push the reading down.
| Dashboard says | Legal range | Typical GPS |
|---|---|---|
| 50 km/h | 41 to 50 | 47 to 49 |
| 80 km/h | 69 to 80 | 76 to 78 |
| 100 km/h | 87 to 100 | 94 to 97 |
| 130 km/h | 114 to 130 | 123 to 127 |
How to check your speedo properly
- Find a straight, flat road with open sky. A motorway is ideal. A tree-lined lane isn't.
- Mount the phone where it can see sky. Windscreen or dashboard, not a cupholder and not your pocket.
- Wait until accuracy settles under ±10 m before you trust anything.
- Hold a steady speed on cruise control for at least fifteen seconds. Comparing while accelerating compares two different moments.
- Read both, then repeat at two or three speeds. The percentage error is usually the same at each. A fixed offset isn't.
What about the 0 to 100 timer?
The 0–100 panel times a standing start. It arms itself whenever you stop and records the moment you cross 100 km/h, or 60 mph if you've switched units. Only your best run is kept.
Expect it to be a little slower than the manufacturer's figure, and to vary run to run. GPS gives you about one reading a second, so both ends get rounded, and a hard launch is exactly where that hurts. Road surface, temperature, fuel and passengers move the number more than you'd think.
Speed alerts
Tap a limit under the gauge and the page draws a red zone on the dial from there up, turns the number red and beeps once when you cross it. Set it before you set off. The beep is there so you don't have to look.
Where should I put my phone?
- Give it sky. A windscreen mount sees far more satellites than a phone in a door pocket, and you'll see the difference in the accuracy number straight away.
- Watch out for heated screens. Heated and metallised windscreens block satellite signals. If accuracy is bad with the phone on the glass, try the dashboard.
- Plug it in. GPS with the screen awake is one of the most demanding things a phone does.
- Keep the page in front. Phones suspend background tabs, and a suspended tab stops measuring.
What this can't do
It's a reference, not a certified instrument. You can't use it to fight a speeding ticket, it has no legal standing, and it goes blind the moment you enter a tunnel or a multi-storey. Your car's speedo is still the one you're legally required to drive by.
Frequently asked questions
Is GPS more accurate than my car's speedometer?
Under open sky, yes. GPS lands within about ±1 to 3 km/h. Your car is legally allowed to read 10% plus 4 km/h high, and usually does. In a tunnel it's the other way round completely.
Can I use this instead of my dashboard?
No. Use it to check your dashboard, not replace it. It stops the moment you lose satellites, it depends on your phone staying awake, and only the car's own instrument counts legally.
Why is my 0 to 100 slower than the brochure?
Those figures come from a car in perfect conditions with a professional driver and proper timing gear. GPS also samples about once a second, which rounds both ends. Use it to compare your own runs with each other.
Does it work in a tunnel?
No. There are no satellites underground, so the reading drops to zero and the dot turns red. Your trip stats survive and pick up again on the other side.
Will bigger wheels change my speedo?
Yes. A bigger rolling radius covers more ground per turn, so the car under-reads, which is the one direction the rules don't allow. This is a quick way to find out how far off you are after a wheel change.
Is it safe to use while driving?
Set it up before you move. Start it, pick your alert and put the phone in its mount while you're parked. Never touch it while the car is moving.