With no other changes to the engine, going to LPG will actually lose you a bit of power and mileage (a guy tends to lean on the pedal harder when power is down). Back in the day I drove a courier van here in the city that I had converted to the stuff (was 5-15¢ a litre when gas was going for 80¢). My power was definately a little down from when it was on gas and the system was
very tempermental in the cold.
Have a read here for a bit of info about it
The other big downside to a propane vehicle; finding gas stations that have it. I don't know about down south, but up here it is losing popularity (I think because the savings on the price difference just aren't there, only about $0.25 a litre last time I looked) and the cost of the conversion can be quite high - I paid ~ $2,500 back in the early 90's
and that was with a gov't incentive reduction. Road trips were particularly difficult, there was always a great amount of time spent researching where fueling stations were located and the hours they were open (no such thing as self-serve propane), it really restricted the routes I could take and the hours I could travel.
Oh, and parking in enclosed, heated, or underground garages is off limits too. the whole gas expansion thing can cause a release from the tank. I don't know about elsewhere, but here it is illegal to park a propane vehicle indoors.
I would not drive another propane vehicle myself, the upsides simply do not out weigh the downs for my use.
However, there is huge potential for increased power if you build the engine specifically for it due to a much higher octane rating - plus it runs much cleaner than gas. The shop that did my conversion also raced cars at the local track and ran propane; apparently they built the engines like they would for race gas, but ran super cheap fuel instead.
