OK, I have done a rough check with the GPS unit, bear in mind this is riding while holding the GPS unit in one hand.
Speedo indicated 40kmh, gps showed 37, speedo 50-gps 45, speedo 60-gps 56, speedo 70-gps 62, speedo 80-gps 71. My scoot is a ZX50 so will be different than your people, the top side of the top blue panel in the fuel gauge is 82kmh and my top speed was 92.47 kmh. It seems that going up in tyre size was not enough to correct the speedo error. But, I do have a very nice front tyre for my efforts though, this thing does roundabouts like never before, with much confidence and a little bit of hoon.