LPG is liquefied petroleum gas. It was originally a by product of the refining of other petroleum products including natural gas. As far as converting a vehicle in the US to run on LPG, you must first find a manufacturer of a conversion kit or the individual parts to do so. Also the kit must be certified by the EPA for the year of manufacture of the vehicle. Most manufacturers of the kits will have EPA certification for the conversion because in this environment of emission inspections it becomes more difficult to convert secretly. And newer vehicle fuel delivery systems are computer controlled so the ECM must be modified to run the vehicle properly. Depending on what you do for the conversion, the more must be done to the vehicle to make the conversion possible such as re-jetting the fuel injectors since newer vehicles are fully fuel injected and no longer use carburetors. Some like the newer Fords also have individual ignition coils making it even more difficult. The next issue to consider is LPG storage. LPG is compressed and sold usually by weight. Since LPG has less energy than regular unleaded gasoline, you must store more in the vehicle to travel the same distance. Also the tanks are huge and usually don't fit where a traditional gas tank is. So if you have a large truck, then you have the bed, but if you have a Yaris, you are not going to have much fuel. Next, since the fuel is so cold when released from the tank, it needs to be preheated before entering the engine. So in my opinion, LPG is not a adequate fuel for cars. CNG is best, but you run into the same issue with the EPA.