I found my odometer is very accurate... Thank ***. The trip meter is impressive down to the 0.1km, at least on mine, - I checked that on the offroad trip late last year.
Paul, for you information, the roadside advisory radars are very out of sync with calibration... they don't get regular testing (coz they are not legally required to do so you see).
One time, in particular, when a set of us (a few cars in a cruise) travelled at the same speed, one after the other, with cruise control.... during lunch time the last person said the display show variations up to a few kilometers per hour.
However, your obseration is still granted.... 79kmh in real term could be 71kmh.