Natural gas powered cars...

Im not up on what it takes to burn natural gas in a piston engine designed for gasoline, but to burn propane efficiently you either need to raise the compression mechanically (new pistons) or via supercharging. If a car already has supercharging its possible to raise the boost pressure fairly inexpensively.

It is possible to run propane without raising compression in a normally aspirated engine but you wont get very great power or fuel mileage that way. Propane would need to become mighty cheap compared to gasoline again before it would make economic sense for a person to just slap a propane injector or dual-fuel setup on his car without any other changes.

I know all this first hand because I bought a propane powered box van from an industrial laundry to haul around my race bike in the 80s. The laundry had the van stripped of gasoline gear and fitted with propane gear to run laundry locally. At the time propane was very inexpensive compared to gasoline so it worked out OK economically. I had to carry another tank of propane and carry a campers bible which had listings of every place that sold propane to make it accross country but that was OK. Not long after that propane rose in cost and the abysmal mileage eventually made converting the van to gasoline worthwhile. Too expensive and time consuming to put in 14:1 pistons and too much uncharted territory to want to add a turbocharger or blower to a beast of burden truck and tune it. It was cheaper to buy a gas tank, carb, fuel pump, fuel filters, fuel lines etc and install them than it was to keep paying to feed propane to a motor set up for gasoline. Once it had gasoline it became more powerful and efficient just like it should. Roughly from 7- 9 mpg on propane to 12-16 mpg on gasoline from a small block Ford with a normal van front and an aluminum box.

Howzabout we just burn alcohol instead?