I think there may be another reason my speedo is out by so much (checked today, 135kph indicated, GPS reads 115kph) as I noticed today at 130kph GPS my revs sit at 6000rpm so I read on another forum a guy with the same speed and revs as me at 130kph but he has a 14 tooth sprocket instead of the factory 15.

It seems the speed is read somewhere from the gearbox and if you change to a smaller sprocket it will affect your speed reading as well.