If you are willing to wait a new type of gas conversion will be on the market here soon, it's called 'vapour injection'. There are a few versions around, some better than others, but the better ones have close to a 1:1 consumption ratio gas:petrol. So if you get 12L/100k's on petrol you'll get 12 point something or so on gas. And here's the clincher, you get MORE power.
It only works on fuel injected cars, the injectors are replaced with larger apeture ones so more fuel volume can get in, some i've read about need a second computer to translate to your ECU but the newer generation don't.
One Aussie example is the 'Stealth' system (www.gas-injection.com
) and a guy here in Perth recons he'll be ready to sell a version (not sure if it's the stealth system) within a couple of months.
It's more expensive than current conversions (about a grand extra i think) but with the added economy it won't take to long to pay it back, and with the amount we spend on performance enhancing it's not too bad.
Also you don't have the risk of backfiring and for anyone that's interested it's a fair bit gentler on the environment.
I'm saving up at the moment.