Thanks backpacker, that's a very good point. I was generalizing on engine types so I agree we should factor in leaning and horse power etc.
My car has a 60 litre tank and will do highway driving at 80 mph (80% engine max) for 8 hours. I do a lot of motorway driving which is what sparked my thinking in the first place!
So my car, 134 hp with 314 lb torque, acheives 7.5 litres on constant velocity highway driving at 3,000 rpm.
Conversely, I fly at around 2,300 rpm and use 40 odd litres per hour. While we need to factor in engine max and horse power there is still a huge difference in consumption. Just pondering why?