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Really good stuff. The underflow of these cars I imagine is already really good. Had my car up to put on the exhaust and it is FLAT under there. Didn't see much to cause drag at least compared to the many other cars I've been under.
I'll probably take mine off too though unless I can get them straightened out somehow or a pair of good/cheap replacements.
There is a separation of airflow behind the tires which causes additional drag. A way to reduce this drag is to use a plate or flap designed to divert the air around the tire, thereby, limiting the amount of flow separation behind the tire and reducing drag.
You can get ridiculous mpg gains if you had a massive front one covering the whole front-- aero modders on the forums do it all the time for eco. it just looks ridiculous.
Where did you see this? And ridiculous MPG gains??? I'm skeptical.
If you think in terms of air flow its like adding wheel skirts to reduce under car turbulence and wind nose. If air is not striking the leading edge of the tire it can improve mpg's.
I'd guess it matters more at speeds above the silly restrictions on US highways.
At some level they also catch rocks and debris reducing the chance them going under the tire and being shot into the air as projectiles.