Wow - interesting to hear this result as I was considering doing something with the rear wheel openings on my xB as well as a rear belly panel but now that this comes up it seems maybe some wind tunnel testing of a spinning tire is needed to see what actually happens to air flow around a tire. Guess the best thing would be smooth wheel covers? Even that may turn out to be false and funny if having air pumping outwards by bladed spokes resulted in lower drag.
I had a similar experience with my Metro, which I attributed to an increase in "parachute effect". The covers were piling up air that would normally be exhausted through the wheelwells.
A rear belly pan solved this. I intend to vent the bumper to further release any accumulated air.
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In another thread here I saw someone had not done just a grill block, they'd added louvered vents to the hood so air could exit the engine bay through the hood.
If you haven't cut your skirts up yet, maybe you could try adding louvers to the rear area. I've also wondered if a more open area behind the wheels could help.
Looking at this blog they put in some ducts to vent the high pressure from the rear wheels. How open are the wheel wells towards the chassis? Piccies would help...
Unlike the rest, I've been happy with my rear wheel skirts. They added a couple mph to rolling down a nearby hill in neutral starting at 40mph. I replaced the original duck tape with fake leather scrapes and galvanized wire.These do have a space in the back half for air to escape. I didn't plan it that way and now don't plan to fix it.
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