I weigh about 225, and I've only been able to put 50 mi. on my bike (yes, I know it's sad, but I'm workin' on it!). That 50 mi. was put all on the first tank, which I gather to be about 1/2 gal. So, I'm guessing I'll get between 90 and 100 mpg.
I've got a fairly stock 48cc, and this last tank I ran up 61 miles before it died out (2 blocks from home, up hill too ... I actually had to pedal somewhere beyond starting my bike today :???: ). This was with the 1/2gal tank topped off and an average speed of 22.1mph.
As soon as I go fill up tomorrow (or tonight I suppose, I don't really have anything else to do, and Circle K is just down the street) I can get an exact figure. But after about 8 months of use and just under 1k miles I'm getting around 120mpg.
Side note, the old Schwinn speedometer works really well for me (manual found here). When you install it just make sure to put it in the center of your handle bars to shield it from the electromagnetic interference of the engine ... for the first few months my speedometer cut out whenever I started the engine and I had no clue why. The computer was on the right side of my bars; repositioning it solved the problem though.