Good points except I don't get 60-70 mpg "normally" - I'd estimate 80-100 mpg would be average (80 on days when I get more "stop and go" type traffic)
I think a more accurate efficiency challenge would be to look at gas usage over a period of time anyways (like I said elsewhere in this thread, looking at things from too close a perspective doesn't give an accurate picture) and if you did a month or two's usage of gas... I would suspect you'd see a fair amount of difference.
Actually thats a challenge we COULD do (using the honor system and expecting everyone to report accurate results) - simply put we record our mileage over a months time and also record how much gas we put into the moped/motorcycle (maybe even two months to get a more accurate sampling) That would give a pretty good picture... presuming similar riding habits. (obviously it'd only be fair to compare similar riding environments... urban against urban, rural versus rural)
And I'd agree.. two strokes (more modern ones are a bit better but still lacking) on a strictly CC compared to CC basis are not as efficient.. however I'd still bet a 49cc fairly up to date moped would beat out a larger motorcycle driven downtown. Takes more gas to get a bigger engine and frame up to speed after all, and I think that would more than balance out any efficiency issues. Mind you a 50cc 4 stroke motorbike would likely be an entirely different animal :)