My wife laughs when I suggest that keeping my HCH II shiny clean and waxed might actually help MPG. I figure in the city it probably makes 0 difference, but on highway trips, I expect it would help. Has anyone done experiments to see if this is true and to what extent?

 

I cant remember the exact percentage, but I was told by an engineer in the automotive industry that the coefficient of drag increases when the car is dirty. If i remember correctly it was over 10 percent increase (for some reason i am thinking 20 percent) in the coefficient of drag.

I don't know what this translates to in real fuel economy discrepancies, but I suspect its pretty low, but probably significant enough to measure.

 

So in the grand scheme of things, I wonder if this offsets the total energy cost of actually washing the car.

Maybe I need to figure out how to add a sail to my roof

 

You're not the only one with this experience! People have mocked me when I claim to see a connection between mpg & clean/dirty state of car exterior... but I know it's happening...

 

Good to know! What I'm more interested now in is how you're getting 52.4 on average My best tank so far (with waxing before each direction) was a 600 mile round trip at 54mpg.

 

Keeping a good coat of wax on the car probably helps a little bit, though twuelfing's 10% seems a bit extreme to me. Besides, there are better reasons to keep the car clean and waxed. Looks good, and protects the paint and metal.

 

i am not claiming a 10 percent increase in fuel economy. I am saying a 10 percent decrease in the coefficient of drag vs a dirty car.

the coefficient of drag is a small part of the entire equation that results in gas mileage. Sorry if i was unclear. also i am probably wrong, but i was told there is defiantly an effect.

 

Clean has better coefficient of drag than dirty, and clean-n-waxed is even better.
That 10% reduction in drag sounds reasonable to me. It probably translates to less than 1 MPG. But hey, it IS a plus

If you wanna skimp on the effort, keep just the front end (& mirrors) waxed. Those areas would do the most good

 

twuelfing, that makes a bit more sense

 

For highway driving at least, aerodynamic drag accounts for over half of the vehicles fuel consumption, so a 10% reduction in that is very huge, that's as much of a change as Honda achieved in the Civic Hybrid by sealing the bottom of the car up with plastic panels, choosing flat rims, and lowering the ride height, all of which are very significant design changes by comparsion to a carwash -- those exposed exhaust pipes, bolts, and support bars that are underneath the car for example, must certainly be more of an impedance to airflow than a small amount of dirt over paint, and covering those up with smooth panels is less than a 10% difference in drag coefficient.

There will definitely be some difference, but I expect it to be very low.