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I got ~6% as well, in the winter when the denser air has more drag. I already had rear wheels skirts and partial front wheel skirts; so yes smooth wheel covers matter.
They need to be flat (like "coupe" pizza pans/trays) to be more effective. I first had domed stainless steel covers, and I saw an improvement going to plastic covers with (all but one) opening covered in foil tape, and even slightly better with pizza pans pop riveted to the same plastic covers (with no holes).
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