Quote:
Originally Posted by 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).
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You are right about the gap reduction. An interesting thing to note is that if all wheels were also blocked from the inside, the 17 inches and 19 inches wheels would do better than the steel rim. The reason for larger taped wheels to perform less than the 15 inches steel wheel with cover is that the disk brake cover most of the wheel opening from inside airflow in the smaller one, wheras there is a huge gap on the larger diameter wheels which increases ventilation resistance a lot. It would have be interesting to see them test something like a 205/75R15 with a steel cover. I'm pretty sure that would have been the best.
The tires tested were 195/65R15, 235/45R17 and 235/35R19.