If anyone wants to be a crazy,
here is an excellent thread about extreme weight loss. The car is an Acura TL, but I'm sure most of the things outlined can be accomplished on most cars. He lost over 700 lbs!
You can reduce vehicle weight by about 40 lbs if the fuel tank is only filled to half capacity.
Also interesting to note; although the car is driving in stop and go rush hour traffic, and over 700 lbs have been removed from the car, the driver only gained 1.5 mpg from the extreme diet. Removing weight has a very negligible impact on fuel economy.