Originally Posted by CanAutM3
Wheel diameter does not have a direct impact on 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 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
Our 20" VS8.2 setup weighs less than the OEM 19" setup. You will not loose power with this setup. Wheel weight will have a deeper effect on performance than size will. Our M3s are a night and day difference betwen 18", 19" and 20" wheel/tire setups. Weight is the biggest weakness of the E9X M3. Any change to it cause a performance gain or loss.