In using my GPS to determine how off my speedo was (it was actually replaced when I purchased the bike) I found that up to 100kph if I take the KPH reading and divide it by 2 I have an accurate MPH speed. so, at 60KPH I'm doing 30MPH, at 80 I'm doing 40, and at 100 I'm doing 50. But, after 100kph it gets a bit wonky. At 120kph I'm actually doing 65.
I use the KPH reading because the xciting that I have is biased towards KPH and I'm old so I have a hard time reading the small red mph numbers...lol.