I live in the UK. I recently bought a used car, which was an import from Japan. I am not the first UK owner. At the time of import, some conversion/adaptation took place so that the information displayed on the dash (speed, odometer reading, trip meter) is shown in miles (Japan uses km).
I recently had a chance to compare these readings with an independent GPS-based application running on the vehicle head unit. What I noticed is this:
- Speedometer shows the speed slightly above that of GPS at low speeds, but slightly below GPS at high speed (e.g. vehicle's 70 mph corresponds to about 72-73 mph in GPS app, yet vehicle's 30 mph correspond to about 28 mph in GPS app)
- Trip meter shows readings slightly higher than GPS. E.g. on one recent trip, GPS app showed total distance travelled of 19.8 miles, while vehicle trip meter showed 20.4
- Vehicle odometer shows about the same difference between in distance travelled as trip meter
I connected a bluetooth OBDII unit to the vehicle's port and paired it with the app running on the vehicle head unit. One of the parameters the app shows is the speed difference between ECU and GPS, updating it in real time. As I accelerate from 0 to 70 mph, the value of the difference goes gradually from 3 to -3.
I have done such comparisons several times at different locations.
So, my questions are: is this behaviour normal? And if not, then should I worry about it?