Washing your car = 7% better gas mileage

I came across a tip that claims if you wash and wax your car, you can improve your gas mileage by up to 7%, which based on today's prices would be about $.21 per gallon.

Not too shabby.The basic idea behind it is, after washing and waxing your car, the surface of the vehicle is much more likely to be nice and smooth, therefore, it should be more aerodynamic than when you have lots of bug guts smeared all over.

It looks like a sound theory, but I don't know if simply cleaning your car will increase your fuel economy that drastically. At the very least, your car's going to look nice? Source.