2 of the 4 Buicks I own are converted to LPG. Not a environmental, but rather a financial issue though; regular gas here is about $9.00 p/gln and LPG $ 2.20. Converting costs about $1500, so within a few thousand miles the installation will be earned back.
LPG is available at about 80% of the gas stations here.
The reason it is so popular here is because of our (road)tax system. Monthly tax paid is depending on car weight as well as on fuel used. Petrol is the cheapest, then diesel and then LPG. You pay the most for LPG, but the fuel itself is the cheapest, about half of petrol. So it actually depends on the amount of miles you drive, what is the most economical fuel. Because of its high tax, but low liter price, LPG is affordable with high milage.
Now comes the trick. Cars older then 25 years, considered classics, are free from roadtax!!!
Most of the US classic cars here have a US made LPG convertion (Impco). Those are better suitable for larger engines and easy to maintain.
LPG convertion exist of a few main components:
-convertor (liquid LPG into gas; it needs warmth from the cooling system: see photo hoses from and to heater core)
-air filter and mixer (gas and air)
25 gln tank (which can only be filled for 80%; safety regulations):
Switch for LPG or regular gas (so you can use or gas or LPG, the LED's indicats the filling of the LPG tank):
Engine bay; on the left inner fender there is the convertor and on the carb the mixer and air filter: