If you look at
moped2.org they have a listing of laws by state and some of those laws group 50cc scooters in the same class as mopeds, the best option of course is to go to your department of motor vehicles and ask them for the pamphlet on scooters.
scooters2.com also has some information, but I haven't spent any time looking around to find out how good it is.
MopedArmy.com is awsome for information on mopeds, if you have questions about a scooter you will be informed that the site is not called scooter army.
For the most part, most states classify a 50cc scooter as a moped (altho mopeds have pedals) and put a speed limit on them that will vary by state, 25mph is common my state allows 30mph, automatic transmissions are also commonly required to make them simpler to operate as vehicles of this class often just require that you have a normal drivers license, at least in my state anything above 50cc engine size requires a motorcycle license.
I know more about mopeds then alot of the scooters as I work on most of the mopeds that are part of the local moped gang, and I also often get asked to help fix scooters and mopeds when they come in to the motorcycle shop that is down the road.
My Tomos Arrow (2005 50cc scooter) is supposed to get 100+ mpg, I don't have solid numbers because I only have 50 miles on it.
My old (sold last summer) Tomos Bullet (1982 moped) got 80-90mpg with an after market exhaust that gave it a big power boost for climbing hills, and raised the top speed to 35mph.
I also sold a Honda E-lite 50cc scooter last summer, it was a little sluggish compared to my Tomos's when going up hills, on flat and from a stop wasn't bad, gas mileage on that was around 90mpg.
I've ridden a friend's 250cc 2005 Vespa, claims to get around 75-80mpg with it, has a top speed of 80mph, requires a motorcycle license, for an engine that size I would prefer to have a motorcycle, but the vespa has fuel injection so it requires less matence.
If I was to recommend a new 50cc scooter, I would say to look at the Honda Ruckus (never ridden one, but it's a 4 stroke Honda), a Sachs Madass (never ridden that either, but has a 4 stroke 50cc Honda dirt bike engine), or any of the non-china made Tomos mopeds (you can get them with pedals, or foot pegs), apparently the new Tomos scooters (Nitro and Velo models) are made in china and their quality dropped when that happened.