I'm not sure of the situation in the USA but in Europe the tax regime is such that the Price of LPG for use as a road fuel is half that of petrol (gasoline). My company (www.mintlpg.co.uk) specialises in converting cars to run on LPG hence halving their fuel bills.
We have a customer in the London area who runs a fleet of ten Prius as taxis, all have been converted to run on LPG over a number of years. Result is very cheap environmentally friendly motoring (CO2 is reduced by a further 20%).