Originally Posted by gatorfast
Why would power be affected by wheel size assuming rolling diameter remains unchanged?
Wheel diameter per se does not have a direct impact on inertial drivetrain loss. However, as a general rule, one can assume that a bigger wheel will have more mass and that mass will be distributed farther away from the centre of rotation. Both contribute to increasing the wheel rotational inertia, hence the power required to accelerate the wheel.
As an extreme example, a very light 20" wheel could have less inertia than a very heavy 19". In that case, the dyno would read better with the 20" wheels. However, this situation it is not very likely.
Without knowing the inertia difference between a specific 20" wheel/tire combination compared to a specific 19" or 18" wheel/tire combination, it is impossible to assess the relative loss (unless someone already ran back to back dynos of Vossen 20" vs stock 19" on the same car