there is a episode in top gear (british car show) the guy goes 800 miles in a desiel audi, in short he says take as much stuff that you do not need out of the car, dead bodies, chairs on the roof, etc. If you see a hill comming, gain speed before it, and coast up it, never brake, use no electronics, no ac no heater, nothing. Keep the car in the highest gear possible at the loswet acceptable speed, aka slow lane. you just think really hard about driving slow.