I see diesel as the future. Diesel engines can literally burn vegetable oil as a fuel and are PERFECT for propane conversions in the interim.
Regular gasoline engines have too low a compression to efficiently use propane. They would need to add turbo's or up the compression to better than 12:1 to work really well on propane. On the up side, propane makes for a ridiculously simple engine management strategy. Propane mixes on a demand basis and is gaseous, so you don't need fuel pumps or injection programming. Just a mixer plate and an ignition curve set by a 36-1 wheel and MAP sensor.
I say we need to convert existing car engines to propane/ turbo and really develop diesel engines to their fullest potential. You could then build small/efficient diesel generators into a car and run everything off motors direct fed by the generator. You could even have dual generators with the second being very small capacity to act as a failsafe.
Jose