[vex]

I'm going to be frank here. I think the gains in MPG to be had are going to come in the transmission, differential, and wheels. Increasing the transmission and differential alone will yield a large percentage of the MPG increase. Thinner wheels (reducing rolling resistance) will also increase your gas savings quite largely.

The engine itself could save on MPG by cheating it a little bit. Mixing of various fuels (IE LPG) to decrease the required fuel from the normal fuel source, but that's just ignoring the other fuel costs.

For increasing engine efficiency look at the intake itself. If you lower the air intake via a restrictor you lower the required fuel. This of course comes at a cost of a less horsepower, but you aren't going for hp when you're thinking of MPG.

To regain the hp you can install a bypass which will allow the addition of more air when the TPS is increased beyond a certain rate. This should remedy the lack of power, but again at the cost of MPG. I personally think that's the best bet beyond running on one rotor. Additional saving could be had from ensuring the intake adheres to the hemholtz equation throughout the RPM band. Thereby increasing volumetric efficiency just by running the engine.