What an online odometer actually measures
An odometer answers a different question from a speedometer. The speedometer tells you how fast you are moving right now; the odometer tells you how far you have gone in total. This tool shows both, but distance leads: the trip figure counts up from the moment your first GPS fix arrives, and the gauge underneath is there so you can see the speed the distance is being built from.
The number that matters most is the trip distance. Alongside it you get elapsed time, which runs while the page is receiving fixes, and an average speed derived from the two. Top speed records the fastest reading of the session. Reset clears all four at once, which is what you want at the start of a new journey.
How the GPS trip meter works
Every second or so your device reports a new position. The odometer measures the great-circle distance between each pair of consecutive positions and adds them up. Over a drive those small hops accumulate into a total that reflects the path you actually took, including every bend, rather than the straight line between start and finish.
Raw GPS is noisy, so three filters sit in front of that sum. A fix with poor reported accuracy is ignored rather than trusted. Distance is only added when the device reports genuine movement, which stops a parked phone from slowly accumulating metres of drift. And a jump between two fixes that would imply an impossible speed, the sort of thing that happens when reception returns after a tunnel, is thrown away instead of counted. Without those guards a trip meter left running in a car park would happily invent several kilometres.
Why your car odometer disagrees
Car odometers do not measure distance directly. They count wheel rotations and multiply by a tyre circumference that was set at the factory. Anything that changes the real circumference puts the reading out: tyres wear down by several millimetres over their life, a plus-sized wheel or a lower-profile tyre changes the rolling diameter, and even pressure has a small effect. Manufacturers also bias the calculation slightly so the odometer and speedometer never under-read, which is a legal requirement in many markets.
The practical result is that most cars over-read distance by roughly 1 to 3 percent. Reset your car trip meter and this odometer at the same point, drive a decent stretch, and the gap you see is your car's error. If it is much larger than 3 percent, the tyre size calculator will show how much of that a wheel or tyre change explains.
What people use it for
Mileage claims are the most common reason. If you drive your own car for work and need a defensible figure for an expense claim, a GPS trip meter gives you a number that does not depend on a dashboard you cannot calibrate. It is equally useful when returning a rental car with a mileage cap, or when checking a used car whose odometer reading you would rather verify than trust.
Outside cars, cyclists use it as a bike computer when they have not fitted one, walkers and runners use it to measure an unfamiliar route, and boat and kayak users use it because there is no wheel to count in the first place. In all of those cases the same GPS measurement applies and no calibration is needed.
Accuracy and its limits
On open road with a clear view of the sky, GPS distance is accurate to well under one percent over a journey of any length, because small errors in individual fixes average out. Accuracy falls where the sky does not: tunnels produce a gap that is bridged by a straight line, and tall buildings reflect signals in a way that can add small amounts of phantom distance in dense city centres.
The page keeps your screen awake while it measures, so a drive does not need you to touch the phone. If your browser suspends the page anyway, for instance when you switch to another app, the distance covered during that period is not recorded. For a measurement you intend to rely on, keep the tab in the foreground and reset at a clearly identifiable point such as a junction or a mile marker.
Keeping a trip after you close the tab
The odometer above is deliberately session based. It starts at zero, counts while the page is open, and forgets everything when you close or reload the tab. That is the right behaviour for a quick measurement and it means nothing about your journey is stored anywhere.
It is the wrong behaviour if you are logging mileage week after week. For that we make a free iOS app, GPS Speedometer: Drive Log, which records each trip and keeps the history on your phone so you can look back at what you drove and when.
Frequently asked questions
How accurate is a GPS odometer compared to my car odometer?
A GPS odometer is usually the more accurate of the two. Your car counts wheel rotations and multiplies by a tyre circumference that was fixed at the factory, so worn tyres, a different tyre size, or changed wheels all shift the reading. GPS measures your actual position over the ground and does not care what is fitted to the car. In practice a car odometer commonly over-reads by 1 to 3 percent, which is why a 100 mile drive can show as 102 or 103 miles on the dash. The GPS figure drifts the other way in tunnels and dense city centres where satellite reception breaks up.
Does the online odometer work without an app or account?
Yes. It runs entirely in the browser on the phone or laptop you already have. There is nothing to install and no account to create. The page asks for location permission once, then reads GPS directly from the device. Your coordinates are used only to compute distance on screen and are never uploaded, so the trip stays on your device.
Why does the distance stay at zero when I am parked?
That is deliberate. GPS fixes jitter by several metres even when a phone is sitting still, and a naive trip meter would slowly clock up distance from that jitter alone. This odometer only adds distance when the device reports genuine movement and the fix is accurate, and it discards impossible jumps between fixes. The result is a trip figure that stays honest while you wait at a light or sit in a car park.
Can I use it to check my car odometer for accuracy?
Yes, and it is one of the better uses for it. Reset both your car trip meter and this odometer at the same point, drive a reasonably long stretch of open road, then compare. A gap of 1 to 3 percent is normal and usually comes from tyre wear or a non-standard tyre size. Anything much larger points to a tyre or wheel change big enough to be worth correcting, and the tyre size calculator will show how much a given change shifts the reading.
Can I save a trip or see my past trips?
Not in the browser version. The web odometer is session based on purpose: it counts while the page is open and clears when you close or reload it, which means no journey data is stored anywhere. If you need a running log, our free iOS app, Speedometer: MPH Tracker GPS, records each trip and keeps the history on the phone so you can look back at earlier drives.
Does the screen need to stay on while it measures?
The page keeps the screen awake by itself on browsers that support it, so the trip carries on counting during a drive without you touching anything. If the browser suspends the page anyway, for example because you switch apps or the battery saver takes over, distance measured during that gap is lost. For a long journey it is safest to leave the tab in the foreground.