The problem is... Work = Energy.

Even if the engine is 100% efficient, the fuel only contains so much energy, and can only do so much work. The key IMO to making fuel last longer is to design the car so it takes LESS WORK to move it from A to B. The impression I should be leaving is to eliminate things that resist efforts to move the vehicle. Number one on the list is to make it lighter: WHEW, what a load off! Sure is easier to push it now.

Case Study: Two cars get stuck in mud. Which one is easier to move, the 1800 lb Smart Car, or the 500 lb Clever Car?

Discuss... (waves hands around)

I'd suggest that whichever one is easier to get unstuck from mud is also easier on the gas.