LOL funny for you to say that, just the other day on some motoring programme I saw on television a tip for making your car more efficient is to wash your car free of dirt. I'm assuming the weight factor is what they are getting at.
Well normally the crappy weather here does it for me.
I probably give the outside a decent clean once every 3 months. Inside the car is probably twice a month (I don't eat or smoke in my car) so it is reasonably clean most of the time.