GPS speedometer

How to use the speedometer

1. Put the phone where it can see the sky: a windscreen or handlebar mount, the top of a backpack, a boat's console. Not in a pocket or a bag.
2. Tap Start speedometer and allow location. Wait until the status says the accuracy is good; at first it may read waiting for a good fix for 10 to 30 seconds.
3. Pick km/h, mph, kn (knots, for boats) or m/s. The trip distance follows: kilometres, miles or nautical miles.
4. Optional: tick Warn above and set a speed, and tick with a beep for sound. For night driving, tap HUD mode and lay the phone flat under the windscreen to read the mirrored reflection.
5. Leave the page open. Reset trip starts a new trip; Clear saved data erases it from this browser.

How it works out your speed

Most phones report a speed with every GPS fix. The GPS chip measures it from the Doppler shift of the satellite signals, which is steadier than dividing distance by time, so when the phone gives that number the page uses it and says Speed from: GPS speed. Some laptops, some browsers and Wi-Fi-only positions give no speed; then the page works it out from the distance between successive fixes (the haversine formula on a 6,371 km Earth) divided by the time between them, and says from positions. Either way the number shown is the median of the last three readings, so one wild fix does not flash on the screen.

Worked example: fixes one second apart, each 10 m further east, give 10 m/s, which is 36.0 km/h, 22.4 mph (10 ÷ 0.44704) or 19.4 kn (10 × 3,600 ÷ 1,852).

Every fix comes with an accuracy radius. Fixes worse than ±50 m are skipped and the page says it is waiting for a good fix. The trip only adds a step once you are further from the last counted point than 1.8 times the fix's accuracy (and at least 5 m), and, when the phone reports a speed, only while that speed is above 1.8 km/h. Two readings of the same spot can each be off by the accuracy radius, so a parked phone whose position wanders by a few metres adds nothing, while a moving one still counts every metre, because each step is measured from that last point. In a test with a minute of ±3 m wander at ±5 m accuracy the trip stayed under 5 m; a 300 m route of 30 fixes came out within 2%. Moving time counts only while the speed is at least 1.8 km/h; the average is distance ÷ moving time.

What it cannot do

• GPS speed lags and drifts. The reading trails a real change by a second or two, and it jumps or drops out in tunnels, between tall buildings, under trees and with the phone in a bag or a glovebox. Short bursts of acceleration are missed.
• It is not a certified speedometer and proves nothing. Do not rely on it for a speeding dispute, a race result or calibrating a vehicle.
• The page must stay open and the screen on. Browsers stop GPS in background tabs and when the phone locks; a gap of more than 30 seconds is not added to the trip as a straight line, so the trip is short by that stretch.
• Do not handle the phone while driving or riding: set it up before you move, or let a passenger use it.
• With a poor fix the trip counts in longer straight steps, so tight bends and zig-zags are cut short: a winding walk can come out a few percent under the true distance.
• Below walking pace on a device with no GPS speed reading it shows 0, because a step smaller than the accuracy circle looks the same as standing still.
• Speed is over the ground: on a boat a current changes it, and on a plane the browser may get no fix at all.

Privacy

Your position never leaves the phone. Location is only asked for when you tap Start; the speed, distance and trip are worked out in this page. The unit, the alert setting and the trip totals (distance, moving time, max speed, no positions) are saved in this browser's localStorage, and Clear saved data deletes them. The page sends an anonymous page-view count and one fixed word, speedo-start, never a speed or a coordinate.

Questions

Is there a GPS speedometer online without an app?

Yes, this page. Open it in the phone's browser (Chrome, Safari, Firefox), tap Start and allow location. It reads the same GPS the apps use, through the browser's Geolocation API, and nothing is installed or sent anywhere.

Can I use it as a GPS speedometer and odometer?

Yes. The Trip panel adds up the distance between good GPS fixes, with moving time, average and maximum speed. It ignores the few metres of drift a parked phone shows, and the trip is kept on your device so it survives a reload. Reset trip starts a new trip; Clear saved data erases it.

Does it work as a GPS speedometer for a boat?

Yes: pick kn and the speed is in knots (1 knot = 1.852 km/h) and the trip in nautical miles. On open water the sky is clear, so a phone usually gets a fix of a few metres. It is speed over ground, not through the water, so a current changes it, and it is not a substitute for proper marine instruments.

Can I use it on a motorcycle, bicycle or in a car?

Yes, in a mount. Set it up before you move and do not touch the phone while riding or driving. Turn on the over-speed alert with sound if you want a warning above a set speed, and use HUD mode at night to read a mirrored reflection in a car windscreen.

Why does GPS speed differ from my car's speedometer?

Car speedometers are built to read high, never low. UN ECE Regulation 39 allows a reading up to 10% plus 4 km/h above the true speed, so at a true 100 km/h a dashboard may legally show anything from 100 to 114 km/h. GPS measures speed over the ground and, at steady speed under open sky, is usually the closer of the two.