Generally a dual fuel car that has not been modified in any way to suit gas will use about 12 to 15% more gas to go the same distance it would have gone on petrol. It also gives you slightly less power but you can always switch back to petrol if, in certain situations, you consider that a problem.
If you want the best possible performance and economy on gas then dual fuel is not the way to go. Gas is a higher octane and therefor a slower burning fuel than petrol. This means it should be ignited sooner. If you advance you ignition timing at idle to suit gas, it will over advance at high speed on gas and ping its head off on petrol. Because of this most dual fuel cars are simply set up for petrol and the owners are so accustomed to driving them they don't notice the slight reduction in performance or the small increase in fuel consumption. Gas is usually just under half the price of petrol, the overall savings work out at about 45 to 50% so the whole thing looks worthwhile.
Straight gas with engine modifications is the best way to go and not having the option of petrol should not be a problem unless you do a lot of driving in remote parts of the Outback. A gas engine should have more compression, a modified ignition system, a different camshaft, a cold air intake, any manifold heating system disconnected, a lower temperature thermostat and a few other minor things. Some of those things are cheap to do but others are not and the cost would have to be weighed up against the advantages. The thing that will make the most difference is the ignition and that is a must even if you don't touch anything else.
What I have said so far applies to carburettor engines. I have not had any experience with gas and modern engine management systems. It would be interesting to see what Ford has done with its gas Falcons. I don't know if they are dual fuel or gas only. If dual it should be easy enough to have a second ignition program in the computer that comes in when the car is switched to gas.
With your car comtim, I would make sure everthing is tuned correctly for petrol and then do an accurate consumption test. Make sure what you get is in accordance with what the car is supposed to do. Repeat the test on gas and see if the difference is within limits. If it is not then have the gas system checked and adjusted.
Brian
