Wheel swap and drag coeficients

Well, none of us has access to Toyota's CFD and wind tunnel testing for either wheel. So, there is no accurate answer. However, there is discussion of wheel drag in several aerodynamics textbooks, trade papers, and research papers, so it's not out of the question to draw some general conclusions.

The more of the wheel face is covered, the lower the drag of the wheel will be. This is well established, and according to Gary Eaker, who runs the Aerodyne wind tunnel in North Carolina, the benefit of a closed face increases the closer the wheel is to the outside edge of the wheel housing. For maximum improvement, you would need to close off both the outside and inside of the wheel, but that's impractical on a real car with brakes and tie rods and struts and whatnot. I went as far as you can go on my wheels shortly after I bought the Prius, with a homemade solution:





(Pizza pans from Wally World).

My previous car had steel wheels, and I used a commercially available wheel cover:



I tested the wheel covers on the Prius versus the open wheel (no cover at all, not even the stock plastic cover) and found a 2.4% improvement in gas mileage at 50 mph; it isn't a lot, but it isn't negligible either. But, that's a comparison of the bare wheel versus a full cover; the stock wheel cover will be somewhere in between, but I don't know where, and it's probably similar to the 17", since they have similar coverage and shape.

For BRZ wheels versus the stock 17" wheels, there might be a small difference in drag because of the larger spoke coverage of the stock wheels, but it's probably going to be small enough that you won't notice it tank-to-tank if you're trying to measure it; if you like the look of the BRZ wheels, go for it.