They're mostly behind the wheel and I can't tell ANY difference in mileage, although it might be very slight. The wheel breaks the wind before the molding, for the most part.

I don't call mine "mud" flaps. I call them "tar" flaps (they're really not even "flaps" - more like "moldings"). Everyone will ocassionally drive through tar. In addition, it protects from rock chips. I wouldn't drive a nice car withough them.

'05 Accord Hybrid
'05 Odyssey EX-L
'03 1800 Goldwing
'98 Civic EX (16 yr. old daughter's)