GPS speedometer, explained
How your phone measures speed, how accurate it is, what each feature does, and what to do when no reading appears. Type a word to narrow the list, or jump straight to a topic.
22 questions
First steps
Opening the speedometer on a phone, a tablet or a laptop.
How do I use the online speedometer?
- Open this page on your phone and press Start.
- Choose Allow when your browser asks to use your location.
- Wait a few seconds outdoors. The chip above the dial shows when GPS has found you and how accurate the fix is.
- Pick km/h, mph, knots or m/s. The dial, the trip readouts and the alert all switch together.
- Press Pause to stop tracking, and Reset trip to clear the top speed, distance and time.
The page keeps your unit, speed alert and HUD choices in this browser, ready for your next trip.
Do I need to download an app?
No. The speedometer works inside the browser already on your phone — Safari, Chrome, Samsung Internet, Firefox or Edge — using the Geolocation API built into every current browser. There is nothing to install and no account.
Once the page has loaded, the speedometer makes no further network requests of its own. Your phone may still use mobile data to get its first GPS fix faster.
Does it work on iPhone?
Yes, in Safari and in other iPhone browsers. iPhones report their speed directly while moving, so the reading is steady.
Location Services has to be on, and the browser needs permission. In Settings, go to Privacy & Security, then Location Services, then Safari Websites (or the browser you use). Choose While Using the App and keep Precise Location switched on — without it, iOS gives websites only an approximate area and the speed cannot be worked out.
Keep the page open and the screen on. iOS stops sending location to a web page that is in the background.
Does it work on Android phones?
Yes, in Chrome, Samsung Internet, Firefox and other current Android browsers, on Samsung, Pixel, OnePlus, Xiaomi and other phones.
Turn Location on in the quick settings panel, then allow it for this site when the browser asks. On phones that support it, the speed alert can also vibrate the phone.
Can I use it on a laptop or desktop?
It opens and runs, but most laptops have no GPS chip. They estimate their position from nearby Wi-Fi networks instead, which is accurate to tens of metres and comes with no speed reading.
When a device reports no speed, the speedometer works it out from how your position changes over several seconds, and says so. On a laptop that reading is rough. For a real speed, use a phone.
Accuracy and GPS
How speed is measured, and why readings differ.
How accurate is a GPS speedometer?
Under an open sky it is usually accurate enough to check your vehicle's speedometer against. GPS.gov puts a typical smartphone's position within about 4.9 metres (16 feet) in those conditions, and phones measure speed in a way that is steadier than position.
Accuracy drops among tall buildings, under heavy tree cover and in deep valleys, where signals are blocked or bounce. The reading also trails sudden changes slightly, because phones typically report a new position about once a second. The chip above the dial shows the accuracy of each fix as it arrives.
Why does my car's speedometer show more than the GPS?
Because car speedometers are built to read high, never low: UN Regulation No. 39 lets a new car's dial show more than your true speed but not less. Worn or non-standard tyres move it further. GPS measures your movement over the ground, so it is usually the lower, truer figure.
How does a phone measure speed?
A GPS receiver works out its velocity from the Doppler shift in the satellite signals: moving towards a satellite raises the frequency it receives very slightly, moving away lowers it. That is measured directly, so it does not depend on two positions being exact.
The browser passes that speed to the page, and the speedometer shows it as it is, without smoothing that would only add lag. When a device does not provide one, the speedometer fits a straight line through your positions over the last 8 seconds (3 seconds above 14 km/h, so it keeps up when you speed up or slow down) and reads the speed from that. It only uses fixes accurate to 30 m or better, and it is noisier than a measured speed — the page tells you when it is doing this.
Why did my top speed jump to something impossible?
A bad fix, usually from signals reflecting off buildings, can put you somewhere you are not for a moment. The speedometer filters for this in two ways.
- Fixes with a reported accuracy worse than 30 m are shown but never counted towards top speed, average or distance.
- A reading that would need an acceleration of more than 15 m/s² — about 1.5 g, beyond what a road car can do — is dropped as a glitch.
The top speed itself is the middle value of 3 readings in a row, so a single bad reading cannot set it. In very poor conditions a run of bad fixes can still get through; press Reset trip to start the top speed again.
Does it work indoors, in tunnels, on trains or on a plane?
Not indoors or in tunnels: the satellite signals cannot get through. The number greys out, the chip says the signal is lost, and tracking picks up again by itself when you are back under open sky.
On trains and planes it often works next to a window, with gaps where the signal is blocked. The dial widens its scale as you go faster, up to 1,200 km/h. Follow the crew's instructions about using devices on board.
Units, HUD and alerts
The trip computer, the head-up display and the speed alert.
Which units can it show?
kilometres per hour (km/h), miles per hour (mph), knots (kn), metres per second (m/s). Distance follows the unit you pick: kilometres, miles or nautical miles, and altitude and accuracy are shown in feet alongside mph.
The conversions are exact: a mile is 1,609.344 metres and a nautical mile is 1,852 metres by definition. The speed converter does the same maths for any number you type, including running pace.
What is HUD mode, and how do I use it?
Press HUD mode under the dial. The speed fills the screen in large digits on black — full screen where the browser allows it, the browser window otherwise. Press Exit, or Escape on a keyboard, to leave.
The Mirror button flips the digits for one job: reading the speed as a reflection in a car windscreen. Leave it off anywhere else.
How does the speed alert work?
Switch on Speed alert and type a limit in the unit you are using. Above it, the dial and the HUD turn red and a double beep plays, repeating every 5 seconds until you slow down. On Android phones it can vibrate too.
It clears once you are 1.8 km/h under the limit, so a speed sitting right on it does not set it off over and over. GPS has some error, so set the alert a little under the posted limit. On iPhone, the beep is silent while Silent mode is on.
How are the average speed and distance worked out?
Distance is your speed multiplied by the time between readings, added up. That is more accurate than joining up positions: each position is a few metres out, and those zig-zags add length that was never travelled, most of all at walking pace. Below 1.8 km/h you count as standing still and nothing is added.
Average speed is that distance divided by the time spent moving, so stops do not drag it down. Trip time counts all the time the speedometer has been running, and leaves out time while paused.
If readings stop for more than 5 seconds, in a tunnel for example, the gap is bridged with a straight line between the positions either side, so a winding tunnel can read slightly short.
Does it keep tracking with the screen off or in another app?
No. Browsers stop sending location to a web page while it is in the background or the screen is locked. To prevent that, the speedometer keeps the screen on while it is tracking, on browsers that allow it.
If you do switch away, tracking resumes when you come back. If you were moving, the gap is added as a straight line, so the distance can be a little short.
No speed or a wrong speed
Blocked location, no signal, or a speed that will not settle.
It says location access is blocked. How do I fix it?
Location is switched off either for the whole device or for this site in your browser. Turn it on for both — on iPhone, also turn on Precise Location for your browser — then reload the page and press Start.
It is stuck on "Searching for GPS".
Permission is fine, but no position has arrived yet. Almost always the phone cannot see enough sky: step outside, away from tall buildings, and give it a minute.
The speed stays at 0, or jumps around while I am moving.
Anything under 1.8 km/h reads as 0 on purpose. If you are clearly moving faster and it stays at 0, the device is probably giving the page only a rough position: Precise Location is off on iPhone, or it is a laptop using Wi-Fi.
A jumpy reading usually means a weak signal; the chip above the dial will say Weak. Keep the phone where it can see the sky, not deep in a bag or a footwell.
Road safety, data and battery
Using it on the road, your location data, and your battery.
Is it safe and legal to use while driving?
Treat it as a reference, not a replacement for your vehicle's speedometer. Many countries restrict holding or using a phone while driving. Mount the phone in a holder, start the speedometer before you set off, and do not touch it while moving — or let a passenger handle it.
Always follow the posted limits and your vehicle's own instruments. The speed alert is a reminder, not a safety system.
Is my location sent anywhere?
No. Your browser hands your position to the page, the page works out your speed and distance in your browser, and the positions are discarded when you close it. Nothing is uploaded, and no location history is kept, even on your own device.
The only things stored are your settings — unit, speed alert and HUD mirror — and your light or dark theme, in your browser. Your phone or browser may use its own location service from Apple or Google to find your position; that is covered by their privacy policies.
Does it drain my battery?
Faster than usual: GPS is in use and the screen stays on while tracking. For a long drive, keep the phone on a charger. Lowering the screen brightness helps too, except in HUD mode at night, where a bright screen is what makes the reflection readable.
Press Pause when you do not need it. Tracking stops, the screen is allowed to sleep again, and your trip is kept.
Does it cost anything?
Nothing at all. There is no sign-up, no paid tier and no advertising. Open the page in a browser on a phone, tablet or computer and press Start.
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Still no speed reading?
Our troubleshooting guide walks through location settings on iPhone, Android and computers, and what to do when GPS will not lock on.
Did not find your answer?
Most problems are a location setting or a phone that cannot see the sky. The troubleshooting guide goes through both, and the contact page reaches a person.