50 miles per quart on a 4 cycle 49cc. Mostly trail riding (80%) at about 15 mph with a 36 t rear sprocket and a 4 to 1 ratio gearbox gearbox on a 26 inch bike. Total weight of the bike with me on it is 200 lbs.
I'm getting about 110mpg. 5HP Honda GS160 9:1 gear ratio on 26" bike. Normally run at WOT/45mph down back roads and local in-town highways. Total weight with me and full tank (2.5gallons) is probably 250lbs.
I've averaged about 150MPG since 2010. From a stop, I pedal, and I pedal during the ride if I can. I almost never "floor it" and I don't spend much time over 30MPH.
My bike has a fuel tank that holds ~1 gallon. My setup:
Engine: Huasheng 49CC with SBP air filter and custom flexpipe exhaust, drilled main jet 2 steps larger
Transmission: 5:1 4G T-belt and SBP 4-stroke shift kit to a 3-speed Sturmey Archer hub
Bicycle: Nirve Knuckle with fenders, handlebar bag, Topeak rack, dual Wald 582 baskets