A GPS app and your car’s odometer measure distance in completely different ways, so they rarely agree exactly, and a gap of 1 to 3% on an ordinary drive is normal. The odometer counts wheel turns and multiplies by an assumed tire size, so worn, soft or non-standard tires throw it off by the same percentage every mile. GPS adds up straight lines between satellite position fixes, so it comes up slightly short on twisty roads and loses any stretch where it has no signal, like a tunnel.
On open roads GPS is usually closer to the true distance. In tunnels, parking garages and dense city centres, the odometer wins. The useful trick is reading the pattern of the gap, which tells you which one to blame.
How does a car odometer measure distance? #
Your car counts rotations from a wheel-speed sensor or the gearbox and multiplies them by the tire circumference it was programmed with. It never sees the road, and the tire on your car isn’t quite the tire in the software.
- Tread wear. A tire worn from new down to the legal minimum loses about 2% of its diameter; a common 225/65R17 drops from about 724 mm to 711 mm. A smaller tire turns more times per mile, so the odometer counts more miles than you drove.
- Pressure and load. A soft or heavily loaded tire rolls on a slightly smaller radius, with the same effect on a smaller scale.
- Size changes. Taller aftermarket tires turn fewer times per mile, so the odometer under-counts. Going from 225/45R17 to 225/50R17 makes the tire about 3.5% taller and the odometer about 3.5% short.
- Wheelspin. On snow, mud or gravel, a spinning wheel adds distance you didn’t cover.
Curves don’t fool it. The outer wheels travel further than the inner ones through a bend, but the car’s figure effectively follows the middle of the car, which is the path you drove.
The speedometer uses the same signal, but the rules treat the two differently. A speedometer must never read low, so it’s set a few percent high, while the odometer only has to stay within ±4% either way under the current UN Regulation No. 39. That’s why a car can read 3 mph fast and still log distance within a percent. How far off your own car is, and how to test it, is covered in our odometer accuracy guide.
How does a GPS app measure distance? #
About once a second the phone gets a position fix, each a few metres out, and the app adds up the straight-line distance between consecutive fixes. Good apps filter that stream so noise doesn’t become distance.
Speedometer GPS, for example, only counts distance while you’re moving, ignores steps under 5 m between fixes (10 m once you’ve been stationary), skips fixes less accurate than ±20 m, and throws out any jump over 200 m as a glitch. Those filters stop a parked phone from gaining distance and stop one wild fix from adding a kilometre. They also decide where GPS loses distance, which the next three sections cover.
GPS counts horizontal distance, so it doesn’t add the extra length of a climb. That’s tiny: a steep 10% grade adds 0.5%, and most roads are far gentler. GPS distance on hills and curves has the numbers.
Why does GPS read short on winding roads? #
A straight line between two points on a curve is shorter than the curve. How much shorter depends on how far apart the points are:
| Time between fixes | 90 km/h on a 300 m radius curve | 60 km/h on a 100 m radius bend |
|---|---|---|
| 1 second | 0.03% short | 0.1% short |
| 5 seconds | 0.7% short | 2.9% short |
| 15 seconds | 6.4% short | 24% short |
Those figures apply only while you’re on the curve, so the effect on a whole trip is smaller. With a fix every second, corner cutting costs a road trip a fraction of a percent. An app that saves battery by recording every 15 seconds loses real distance on mountain roads.
The bigger losses on winding roads usually come from the signal. Deep valleys, cliffs and tree cover weaken or block the satellites, and the gaps get bridged by straight lines or not bridged at all.
Why does GPS lose distance in tunnels? #
In a tunnel the phone gets no fixes. When it comes out, the gap between the last fix before and the first fix after is one long straight line. Speedometer GPS treats any jump over 200 m as a glitch and doesn’t count it. At 80 km/h that’s any gap longer than about nine seconds, so a 2 km tunnel leaves your trip about 2 km short. The odometer keeps counting.
Why does GPS read short in slow traffic? #
The same drift filter has a cost. Speedometer GPS ignores steps under 5 m between one-second fixes, and below about 18 km/h (11 mph) every step is shorter than that. A long crawl in heavy traffic, or a slow hairpin, adds less distance than you really covered. At normal road speeds it doesn’t come into play, but a stop-and-go city commute can come out noticeably short.
What pushes the two numbers apart? #
| Cause | Which reads higher | Typical size |
|---|---|---|
| Worn tires | Odometer | Up to about 2% |
| Taller aftermarket tires | GPS | Often 3 to 7% |
| Low pressure or heavy load | Odometer | Small |
| Wheelspin on snow or gravel | Odometer | Varies |
| Long tunnels | Odometer | About the tunnel’s length |
| Winding road with a weak signal | Odometer | Usually under 1% |
| Slow stop-and-go traffic | Odometer | Varies with how long you crawl |
| Tall buildings | Either; scattered fixes can read long | Varies |
| Phone in a pocket or cup holder | Odometer | Can be large |
| Car ferry with the app running | GPS | The crossing distance |
Most mismatches come from a few of these at once. A car on worn tires on a twisty mountain road will show the odometer ahead of GPS from both directions.
How to read the pattern of the gap #
- The same percentage on every open-road drive points to the car. That’s tire wear, tire size or pressure, and it won’t change until the tires do.
- A gap that only appears on certain routes points to GPS conditions on those routes: tunnels, valleys, tree cover or slow traffic. A saved trip in Speedometer GPS keeps the route and a speed chart, so you can see afterwards where the signal dropped or you crawled.
- GPS 5% or more short on an open highway usually means the phone, not the car. It was in a pocket, bag or cup holder instead of mounted with a view of the sky, you switched to another app mid-trip (Speedometer GPS doesn’t track in the background), battery saver or approximate location was on, or the trip started before the phone had a good fix. Our guide to getting accurate readings from a GPS app covers the setup.
- A sudden jump in the gap after new tires is a size change. If the odometer now reads 3% or more low, see speedometer error after new tires.
Which number should you use? #
- Service intervals, warranty and lease mileage: the odometer. It’s what the car, the dealer and the paperwork use, whatever its error.
- The true distance you drove on open roads: GPS with a good fix.
- Planning a route: the map’s route distance, measured along the road centreline before you leave. It’s a third number that neither the odometer nor GPS will match to the decimal, because your real drive includes detours, parking lots and wrong turns.
- Work or tax mileage: whatever your employer or tax authority accepts, used consistently. Many expect odometer readings at the start and end of each trip.
Frequently asked questions #
Is my car’s odometer or GPS more accurate? #
On open roads with a good signal, GPS usually comes closer to the true distance, because it doesn’t depend on tire size or wear. The odometer is more reliable in tunnels, dense cities and slow traffic, and it doesn’t depend on a phone.
Why does GPS show fewer miles than my odometer? #
Usually worn tires making the odometer read a little high, plus GPS losing distance in tunnels, on winding roads with a weak signal or in slow traffic. If the gap is the same percentage on every open-road drive, it’s the tires.
Why does my odometer show more miles than Google Maps? #
A map’s route distance assumes you follow the planned line exactly. Real driving adds detours, parking lots and lane changes, and worn tires make the odometer over-count a little. A gap of 1 to 3% is normal.
Do curvy roads add miles to my odometer? #
Not in any meaningful way. The outer wheels travel further than the inner ones through a bend, but the car’s figure effectively tracks the middle of the car. On curvy roads it’s GPS that tends to read short.
Do bigger tires affect my odometer? #
Yes. A taller tire covers more ground per turn, so the odometer and speedometer both read low by about the same percentage the diameter grew. A GPS comparison on the highway shows it clearly, and a dealer or programmer can usually correct it.