Wax car...lol. I have read things that claim washing/waxing your car helps improve mileage. Now I cannot confirm nor deny this, but heres what I found
"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." One link claimed 7% increase by doing this..
Regular tune-ups, clean air filter, go the speed limit. What transmission does the car have in it?