Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: What is an odometer? It's the instrument that records the total distance a vehicle has travelled. How it works depends on the type. Older mechanical odometers turned numbered drums through gears driven by the gearbox. Modern cars count pulses from wheel or transmission sensors and store the total electronically. GPS odometer apps add up the distance between successive location fixes.
TL;DR
- Mechanical: cable and gear-driven drums.
- Electronic: sensor pulses × calibrated distance.
- GPS: sum of distances between fixes.
- Car odometers often read slightly high.
- GPS can miss distance in tunnels.
What is an odometer? Mechanical odometers
A flexible cable from the gearbox spun a series of worm gears. The ratio was chosen so that one turn of the last drum equalled one-tenth of a mile or kilometre. Wikipedia's odometer article describes the history back to Roman-era designs.
How odometers work in modern cars
Electronic odometers count pulses from the ABS wheel-speed sensors or a transmission sensor, multiply by a programmed distance per pulse and store the total in memory, often in more than one module to deter tampering. Because they share calibration with the speedometer, they share its small over-read.
How GPS odometers work
An app such as Speedometer PRO records a position every second or so and adds the straight-line distance between consecutive points. Short gaps are fine; long signal loss (tunnels) cuts corners. Noise at standstill can add phantom metres unless the app filters it.
Sources of drift
| Type | Drifts because of |
|---|---|
| Mechanical | Gear wear, tyre size |
| Electronic | Tyre size, wear, calibration |
| GPS app | Signal loss, sampling, noise |
Checking your odometer
Drive a known distance, such as between motorway markers, and compare. Or record the same route with the odometer and car together. Read measure distance driven.
Mechanical to digital
Early odometers used gears driven by a cable from the gearbox, turning numbered drums. Modern cars count pulses from a sensor and store the total electronically, often in more than one module.
Why odometers can be wrong
Wheel-based odometers inherit tyre-size errors, typically a few percent at most.
Odometer law
In the US, federal law prohibits odometer tampering and requires disclosure on transfer; NHTSA runs consumer guidance. See odometer fraud.
Related: Odometer guide: odometers and trip meters · Odometer app: total distance from GPS · Odometer vs trip meter · How to measure distance driven · Odometer fraud: how to spot it · Best odometer apps · Trip meter: duration, distance, average and max speed · Trip tracker: your past trips in one list
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- Odometer (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Odometer Fraud (NHTSA): nhtsa.gov (accessed 26 October 2026)
- GPS Accuracy (GPS.gov): gps.gov (accessed 26 October 2026)
- Odometer fraud (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
FAQ
Can an odometer be wrong?
Yes. Tyre size changes, wear and faults affect wheel-based odometers, and GPS odometers lose distance when signal drops.
Do digital odometers store mileage in more than one place?
Many modern cars store it in several modules to make tampering harder, though methods differ by maker.
Does a GPS odometer need internet?
No. GPS works without data. Maps may need data to load tiles.
Why does my app show distance while I'm parked?
GPS noise can make a stationary phone appear to move a few metres. Good apps filter this.
What does odometer mean?
From Greek words for 'road' and 'measure': an instrument that measures distance travelled.