In a controlled environment, I am sure that the dinner plates and skinny tires give better aerodynamics than the 5 spoke wheels, but in actual operation there are so many variables at play that the delta on fuel economy loss is probably not that great. I have not seen any difference in my avg. fuel economy between the two types of tires.
But then again, I am still getting the hang of driving the car for economy.
The 5 spoke wheels sure do give me more SPMs than with the dinner plates.
*SPM - Smiles Per Mile