[MetroMPG]

Those aftermarket rims are "air blenders"!

It's interesting to note that the exposed 19 inch rim is the worst case, but when masked, it's the 2nd best case - better than the masked 17 inch rim.

To me that suggests the shrinking gap between tire and wheel arch (you can see the progression in the photos) may be having a beneficial effect. And it may be even more significant if they didn't control for tire width as well (they apparently didn't control for overall circumference, so I'd be surprised if they did control for width).