Had a coworker back in '94 that ran his truck off either gas or propane, depending on the fuel prices. He saved alot of money with that setup.
This is somewhat off topic, so I apologize.
Now, I personally want to see liquid propane fuel systems that can be retrofitted to existing engines. LOTS more power versus gasoline. As it expands in the cylinder it kind of creates it's own boost. In testing, a 500 HP Chevy smallblock was bumped up to 750 HP with no other modifications. That's awesome. The owner of Mother's Car Care Products is a big proponent of liquid propane, and/or liquid natural gas. We have huge reserves of natural gas here in the US, so it just makes sense to fuel our vehicles with it.