I'm pretty sure Phil has posted before that a convex wheel cover/shape is better. I'll search around to see what I can find. Can you point to a source for flat being better?
EDIT: found the post - it's not conclusive.
Quote:
Originally Posted by aerohead
I dug into my mess.I'd read an explicit argument somewhere about the merits of the convex vs flat covers,however did not,or could not copy the article,so I don't have it.
So all I can do is default to Shinella's R&D of the Trans Am at Lockheed,Marrietta.They got numbers for an open steel wheel,a vented flat wheel cover,the same cover taped closed,and Shinella's
secret weapon discs which shaved another 0.003 off the Cd.
The production covers are convex and also vented and no data are presented for them,nor are the secret covers shown.
Since Ultralite sports a shallow convex,minimally perforated cover,and at 100-mpg,being fully optimized for low Cd,I would be comfortable with the notion that the crown of the convex cover has a distinct advantage over the flat.
We're splitting hairs though I think.
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