i used to have a couple of scooters in the early/mid '80s, a Honda passport C70 and later a Yamaha 180. the passport got well over 100mpgs (about 120, if memory serves me well) and the 180 got about 80mpgs. i was looking into buying a scooter capable of 50mph with good mpgs. in the summer months i average 50mpgs with my smart in daily usage. using the lofty $4 a gallon gas price, if i were to go to say a Honda elite 110 that got a smidge over 100mpgs, at a price of $3K, plus insurance... and for the 200 miles a week i usually drive, i'd save a bit over 2 gallons a week, at $4 a gallon, i'd save $8 a week, for the "riding season", call it what you want... i'm near chicago, it could be 6 months, i don't really care to ride when it's below 50F outside. i'd save about $200 a year in gas riding the scooter half the year... i'd have to own/ride it 15 years to make up the savings in gas, i'd also have a loss of comfort and safety the smart offers, no audio, no rain protection, and that's not including insurance, parts and maintenance.
IMHO, a smart beats out a scooter capable of 50mph any day of the week. i was shopping for scooters online for giggles and thought about a possible advantage, and when i'd see a benefit. scooters are fun on sunny, warm days, useless when it snows, or when you're uncomfortable. i've been carrying a "motorcycle rating" on my drivers license for the last 25 years without using it, in the hope of someday getting another scooter. that costs me an extra $3 a year. my license is up for renewal in 2 years, i'll drop the "motorcycle" rating. owning driving a smart is more economical year round than any other sub-compact car for half a year and a good scooter a half year.
with the smart, you have safety, storage, unlimited range (cruise control!!!) audio and all weather capabilities. if your main car is an SUV that gets 15mpgs, you might want to try a scooter, but with a smart as the primary vehicle, you're better off "smart" and not "scooter".
I own a smart and a scooter (50cc) and I ride the scooter year round here in Colorado (as long as there is no snow on the ground or it rains... We have 300 days of sun here in Denver). 50 ccs are considered "motorized bicycles" and allowed to park on the side walk. I get about 70 mpgs, so that even beats the smart, and I save about $100 to $150 per month on downtown parking. The scooter cost about $2,500, ten years ago. Just for the parking cost I could buy a new one every two years. Only riding a bike would be cheaper, but it also takes longer and I would need a shower and change after getting to work :).