I have my AU I6 on duel fuel and usually get 350kms in city driving from approximately 55-60 litres which seems to be less than most people get here? But then my car is heavier with 17" wheels and bodykit and used mostly during peak hour, and also is Tickford which is supposedly thirstier? The tank is supposed to be 68litres but even with the trip computer beeping on 5km left I never can fit anymore than ~60litres.
I would look at the amount of kms you do a year and how long you plan on keeping your car so the initial outlay pays off? Fill up when it's at its lowest in the mid 40's and it costs around $26 to fill up. Cheapest petrol is low 90's and will cost around $55 to fill. Then you say the gas uses a third more so around $35 to get the same kms as for $55 of petrol, so in the end you still end up saving around $15-20.
"If this were a dictatorship, it would be a heck of a lot easier -- just so long as I'm the dictator." -- George W. Bush (December 18, 2000)