GPS speedometer: your real speed from satellites

An online tool for measuring speed with GPS. It shows your current speed in glance-sized digits and tracks maximum, average and distance covered. The calibration tab compares your car's speedometer against true speed using the UNECE R39 tolerance.

How to measure speed with GPS

1
Allow location

Open the page on your phone and press "Start the speedometer" — without satellite access there is nothing to measure.

2
Wait for a fix

Under open sky the first reading takes up to a minute. The status strip shows how strong the signal is.

3
Start moving

Speed appears on the dial. Maximum, average and distance keep themselves.

Read your speed from satellites and check it against your dashboard

An unfinished drive is waiting

🚗
Turn on the GPS speedometer

This tool needs location access — without it there is nothing to measure speed from.

  • 📡Speed comes straight from the GPS receiver, which solves it from the frequency shift of the satellite signal.
  • 🛰️It is sharpest under open sky. Tunnels, forest and dense city blocks weaken the signal.
  • 🔋Your screen stays awake while the page is open, so it will not go dark mid-drive.

Location access was declined

Your browser blocked location for this page, so there is nothing to measure speed from. Here is how to give it back:

    No satellites yet

    The first fix takes up to a minute under open sky and may never arrive indoors. The tool keeps listening — step outside or move to a window.

    Point your phone camera at the code to open this page there.

    Approximate location is switched on

    Android is placing you within hundreds of metres, so speed will be coarse and late. Open your browser app settings, allow "Precise location", then reload this page.

    Waiting for signal ⏸ Paused — tab hidden
    0
    Max
    0
    km/h
    Average
    0
    km/h
    Distance
    0
    m
    Time
    00:00
    moving
    Heading
    —
    Altitude
    —
    Fixes
    0

    Compare what your dashboard shows against true speed from GPS. Check it while holding a steady speed on a straight road — have a passenger type the number, or memorise it and pull over.

    True speed right now
    — km/h
    Type the number off your car's speedometer at the same moment — in km/h.
    No track yet. It records while the page is open and the signal holds.
    The track lives in this tab's memory and disappears when you close the page. Download it if you want to keep it.
    km/h
    Going faster than this turns the number red. Always set in km/h, whatever unit you display. Zero switches the alert off.
    Alert sound
    A short tone when you pass the threshold, at most once every 2.5 seconds.
    Keep the screen on
    Holds the screen awake while this page is open.
    Smoothing
    Steadies the jitter at constant speed while letting real acceleration through without lag.
    Mirrored HUD
    Flips the digits so the windscreen reflection reads the right way round.

    Why your dashboard disagrees with GPS

    When GPS reads lower than the dial in your car, both numbers are right. UNECE Regulation No. 39 — the rule the EU makes binding through Regulation 2019/2144, and the UK through Construction & Use — forbids a speedometer from reading below true speed, and allows it to read above by 10% plus 6 km/h. So at a genuine 100 km/h your dashboard may legally show up to 116 km/h, and that is a healthy car, not a fault. Read it backwards: when the needle sits on 100, your true speed is somewhere between 85.5 and 100 km/h. Manufacturers deliberately aim high in that window so they can never breach the floor. The Calibration tab works out your car's own figure.

    0
    km/h
    Lay the phone on the dash, screen up. The reflection reads after dark; in daylight the sun beats the screen.

    Features

    Big digits and a dial
    Speed reads in a single glance, and the needle rescales itself from walking pace to train speed.
    Dashboard calibration
    Enter what your car shows and get the gap as a percentage, judged against the UNECE R39 allowance.
    Trip statistics
    Maximum and average speed, distance and moving time. A single signal glitch cannot poison your maximum.
    Track export
    Save the route as GPX for Strava, Garmin Connect and OsmAnd, or as CSV with speed, altitude and accuracy.
    Four units
    Kilometres per hour, miles per hour, knots for the boat, and metres per second — switchable while moving.
    HUD mode
    Fullscreen mirrored digits for a windscreen reflection, with the screen held awake.

    Frequently asked questions

    Why does my car speedometer read faster than GPS?

    Because the law requires it. UNECE Regulation No. 39 forbids a speedometer from understating your speed and permits it to overstate by 10% plus 6 km/h. At a true 100 km/h the dial may legally show up to 116 km/h. Manufacturers aim high within that window to be safe, which is why a 5-10% gap is a healthy car rather than a fault.

    Is GPS more accurate than a car speedometer?

    At a steady speed on a straight road under open sky, GPS is closer to the truth. The receiver solves speed from the Doppler shift of the satellite signal, which is orders of magnitude more precise than dividing distance by time. A car speedometer counts wheel rotations and is deliberately biased upward. GPS does lag during hard acceleration and braking, because it updates about once a second.

    What is the legal speedometer tolerance?

    Under UNECE R39, the gap between the indicated and the true speed must fall within 10% plus 6 km/h for series-production cars and trucks, and the indicated speed may never be lower than the true speed. Motorcycles get a wider allowance of 10% plus 8 km/h. US passenger cars are not regulated on this point at all — the federal standard was withdrawn in 1982.

    Why does it show 0 km/h when I am moving?

    Almost always the signal is not reaching you: a metal-bodied train carriage, a tunnel, an underground car park or dense high-rise. A window seat usually fixes it. If the status strip reports accuracy in the hundreds of metres, Android is set to approximate location and needs switching to precise.

    Does it work on a train or a plane?

    Yes, as long as the receiver can see satellites — a window seat on a train normally works. The tool accepts speeds up to 648 km/h, which covers high-speed rail and airliners. In the middle of a carriage or in the aisle the signal often drops out.

    Why does it pause when I switch tabs?

    That is how browsers work: the W3C standard stops delivering positions to any page that is not visible on screen. No website can work around it, so rather than quietly freezing on a stale number, the tool tells you it has paused. Keep the tab open — the screen will not go dark while you do.

    Where does the recorded track go?

    It is held in this tab's memory while the page is open and is gone once you close it. To keep a route, download it as GPX or CSV from the Trip tab.

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    Who is this tool for?
    🏃 Sports & Athletics ✈️ Travelers & Tourists
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