Clean vs Dirty car aero
 

I have heard that a dirty car gets better mileage than a clean car because the dirt creates a rough surface, kind of like a golf ball. But Mythbusters tested that and found thhat a dirty car reduces mileage, but then again they sprayed a half inch thick mud coating on it, which is not at all indicative of a "normal" dirty car, unless you go offroading lol. What do you think, and how big could the difference actually be? Thanks

You'll get a lot of suppositions with a question like that...but hopefully someone comes along with some hard evidence one way or the other.

I'd guess any aero gains are offset by losses caused by extra weight.

Consider safety instead. Bright & shiny seen better by others.

Glass so clean in & out that new cars look bad.

Vision by others::Vision by driver.

I don't think that the dirt weighs that much lol. But then again the difference in aerodynamics would not be much either.

I don't know, I removed a couple hundred pounds of dirt from my company's backhoe yesterday just so I could service it. LOL

A few years ago one of the world rally championship cars had the dirt they cleaned off it after 1 dirt stage weighed. It came in at 23ish kg if I remember right.

Depending on the part of the world/ country you are in ,not washing the road grime off is a shure way to get rust started and that will cut your mileage and safety.

Authoritative enough?

Permalink #26 in Clean vs Dirty Car / Much difference? in 2015.07

clean vs dirty
 

Aeronautical engineer and creator of the Litestar/Pulse,100-mpg vehicle of the early 1980s claimed Cd 0.18 'clean' ,and Cd 0.20 'dirty'.
It suggests that the vehicle had a laminar body.
Sighard Hoerner,who was with Messerschmitt in WW-II,claimed that the rough,olive drag,flat-primer-like paint on their fighter aircraft made no difference to performance.
Rough,clay,full-scale wind tunnel models appear to generate the same Cd as a production,OEM,lustrous paint-finished model of the same car.