top speed has may variables, of which only one is engine size.
the most reliable way is to follow a car or another bike with an accurate speedo and rake it out. once it wont go any faster, wave to the other vehicle and they note the speed it shows.
it all depends on a lot of variables.....
best thing to do is mark off a mile stretch, then have a friend or someone time you...
or you can go out and buy a trail tech off road speedometer, and find out yourself......