On standard tyres the trip meter/odometer is reasonably accurate. Mine is out by about 50m over 5km (1%). I suppose that's substantial given that my car has supposedly done 245,000km - the error could be around 2,450km. For fuel economy purposes, I get up to around 1400km to a tank of fuel, so the error could be 14km - when you do the number crunching, it could be (for 150L) 150/14 = 10.71LPHK or 150/13.86 = 10.82LPHK.
The speedo needle deflects against a spring depending on the voltage sent to it (electronic speedo) or by a spinning magnet inside a shroud against a spring (mechanical speedo). The little bugger of a spring has to be the perfect tension - and if it's within ADR spec (indication > actual and < actual + 10% + 4km/h) Nissan won't lift a finger to fix it. Using your GPS is the best alternative.
