Mach should be accurate as long as you are getting a steady reading, the only other thing that I can think of that could make the reading higher than it actually is is the pulley setup from Config menu, if you had a ratio there on the pulley you are using it could show an increase in speed. To see what pulley you are on look at Settings page and put it to 1 if you dont have pulleys, then on the Pulley Setup from Config menu make sure pulley 1 has your max speed and no ratio set.
Hood