On electronic speedos the over-indication is built into the logic circuit. On one ride,the two Suzuki Freewinds on the ride read exactly the same erroneous speeds and distances compared to the calibrated cyclo computers fitted to the bikes.On analogue speedos the odo was directly driven, the speedo was a magnetic drive, so there was always a difference. I have noticed on my 09 that the cycle computer will read around 5 km's less over 180 km's than the trip function on the bike. The speed readout is often as much as 9 km/h fast.
The cycle computer is calibrated to tyre circumference and the speed shown is accurate.
Most countries adhere to UN ECE Regulation 39. in some way or another: "...no speedometer can read slower than the actual speed. Ever. On the high side, it’s allowed to read up to 10% above the actual speed plus four or six kilometers per hour, depending on the type of vehicle."