Mythbuster solve this question quite well the other day.
I'm not one that keeps a car washed and waxed all the time and I somehow knew washing and waxing never improved my mileage. I wasn't aware that a dirty car improved mileage until I saw mythbusters the other night. I've been putting off washing my '09 FEH
because a month or so ago I was ask to be in a documentary on Hypermiling
the BMW X6 ActiveHybrid. They said they wanted to include footage of me and my '09 FEH
so I put off washing it till I had to drive to Miami for the shooting on 11/6/09. The FEH
got real dirty during this time but my mileage was increasing. About 3pm on 11/5/09 I took the FEH
to the car wash and had it detailed and hand waxed. Since that time I noticed a decrease in MPG
of about 10%. I've never noticed that big of a drop in just washing the FEH
but waxing it really made a difference for the worst. No question in my mind that a dirty FEH
gets better mileage than a waxed FEH.
The '09 FEH is slowly getting back to normal now that a layer of road dirt and dust is starting to coat the wax.