I have a 50 cc Honda Metropolitan, which gets about 100 MPG. The only drawback is the top speed of about 40 MPH. If you need to go faster, look for something in the 125-150 cc range. They still get 75-80 MPG, depending on which you get. And they can do 50-60 MPH.

The other drawback with a scoot is when the weather sucks. Nothing stings quite like a cold rain at speed.