i went rideing sat. for the first time in a while. i got a new trail tech speedo and the speedo said 75mph but i do not think i was toped out because the wind made the bike unstable. i was at the top but i don't think all the way because i did not fell the rev. hit. when i first got the bike i ran my buddy in his stock truck and he said we top out at 81mph. i need help on setting the tire size on my speedo i put in 21.07, is that right for a stock front tire on a ds.please help!

 

The most accurate way to get tire diameter for this purpose, would be to measure from the center of the spindle down to the ground, and then double it.

This method is most accurate since it allows for tire deflection due to load.