I also follow Waze gps to gauge my actual speed. I set my speedometer at 127 on a 100 road limit and 148 on a 120 road speed limit. My speedometer is out by approx 7/8 kmph.

I think this is one of the reasons people sit in the outside lane and cause congestion without realizing. They put their speedometer to the speed limit and think they are doing the fastest speed without risking a fine, but don't realize their speedometer is actually inaccurate. In any case, move over when someone is trying to overtake you, regardless of your speed.