my r33 isnt great on fuel, say 300km to a tank but i dont exactly drive with fuel consumption in mind... price u gotta pay with these cars.
as for lpg... i have a mate with a turbod celica that runs on lpg... fasted car ive been in.
it was the fastest car in QLD in recent year i believe for its size
but personally i would stick to petrol and just use good fuel like BP Ultimate (dont use cheap unleaded)