That's about 50-65MPG, assuming you've run it bone dry. I doubt you have, so it's probably closer to 65-75MPG.
That's about average for a 66CC chinagirl ridden normally with a man-sized dude in the saddle (read: 200lbs at least)
The so-called "sweet spot" is where your engine is around torque peak - where it does maximum work with the least effort. The RPM range for this depends on the engine, intake and exhaust used, and whether or not one has done any porting mods. With a bone stock chinagirl kit, this will be about 3500-5000 RPM.
With a 44T sprocket and 26" wheels, 3500-5000 works out to 15-22MPH. If you stay in this range, you'll find your MPG will go up to about 80-90.