Originally Posted by
Nash_mr2
If the dyno Cylinders are lets say 17" in diamiter and you go on there in your car with 15" wheels - you'll get a power reading of XXX
Then you take the same car, on the same dyno, with the same setup, except change your wheels to 19", then you'll get a different power output reading - this is becuase the mechanical drag that the dyno uses to convert to tourq has been affected by the increased rolling diamiter of the wheels.
Once fitted to a car, a
wheel doesnt have a rolling diameter mate. The Tyre does, and that
should be chosen so that the rolling radius is in fact exactly the same as it always was.