As lewi just posted:I think you meant higher not slower
he signalled when he was doing 60 on his speedo ... When i caught up and held a constant speed behind him it was around 66ish kph
So yes, reading higher than the speed you actually doing. If it were the other way around there'd be a million cases of manufacturers getting sued because a driver was "speeding" a few kph over the limit but they had no idea because their speedo was showing 60 (reading under). Also, as lewi has also pointed out, the 10% speedo error seems about right.
What I want to do is get a popo to radar me. I'll sit at 60 on the bike (which should be about 51 kph for my 13/38 gearing) and see what he says lol.
Still need to do more phone-taped-to-fuel-tank gps tests.