N.E. Lincolnshire UK
Its not just the size of the tank or how rich/lean you are running it its also the nitro content, when I was using 10% coolpower I was able to get flights in the high 20's of minutes from a full tank, also with lower nitro the ammount of methanol goes up, which aids fuel consumption, this is on a standard* SX-H
*Done the cats eye mod and the web mod.
With running lower nitro the engine doesnt get as hot and it can run at leaner settings without wanting to quit, also depending on the exhaust you use you can save fuel there, I use Zimmerman pipes on my helis and have found I get good fuel economy from them (seem to let less fuel out of the exhaust than other brands of pipes)
Before anyone says you must be running it lean, nope, pleanty of smoke, I've had this engine for over two years and the piston and liner are in very good condition, and dont look to be needing replacement any time soon, my headspeeds are 1650 for low and 1850 for high, most of the time spent at 1850 as thats nearer to the powerband of the OS50, which means its also more fuel efficient.
Fuel economy is also affected by how you fly, if you are flying arround doing circuits your fuel economy will be at its highest since you can use transitional lift to let you fly at a lower collective/throttle position than in the hover, and 3D is obviously the worst for fuel economy.
Also it depends on the time of year that you are flying, in the winter because of the cold you can run leaner and save fuel also.
As someone said, not everyone considers a flight 100% 3D and nothing else.