It has to be both, think about it. The wheels move the thing inside that displays both, distance and speed. Otherwise it would need to sensors, one for each. If speed is off by say 5%. so is the distance. Next time you go on a long trip, reset your trip counter next to one of those green Km markers. If you can drive 100Km according to the markers, compare it with the trip journal aboard the car. Should be easy to do. I bet it will be off by the same percentage of the speed. I had mine measured against a police lidar. Nor super scientific, as I tried to maintain 100Km/h according to it (I was reading the speed off the ODBII port to get a digital reading). Same thing happens with the trip counter, Dash-Commander pulls the distance from OBDII but does not match what I planned with TomTom.
Anyhow, happy testing.
View my fuel log 2015 Micra S manual: 7.7 L/100 km ... 36.6 mpg (Imp) ... 13.0 km/L ... 30.5 mpg (US) ...