My ‘67 MG Midget was 1275 cc and a top speed, as measured by Maine State Police, 105 MPH. That was after a port and polish job.
I think the eventual place for vehicles like this will be as a tactical second car. I drive a pick up because I have to do work around the house, etc and need to be able to move furniture, sheet rock and other equipment. However, it gets around 16 mpg in town. Driving to work or the store doesn't require me to hit highways, but the economics of having a second vehicle that gets 30 mpg for the short trips isn't there. That's why most people have bigger vehicles. The person who drives an Expedition to work alone probably also uses it to transport the entire family several times a week. If gas hits $6 per gallon, you'll start seeing more "tactical" vehicles. When gas was fairly cheap, it made more sense to have one vehicle that did a lot of things.
The economy of having one vehicle that does multiple things but gets lesser mileage exceeds the economy of having an extra vehicle that is only for short commuter hops but gets great mileage. Automakers, though, continue to price vehicles as if they will be the only transportation system for a person. Mopeds and motorcycles require skills that a lot of people don't have, and expose the individual to the elements, so they have limited applicability (also, many motorcycles don't get THAT much better mileage than a small car.)
When you'll see these vehicles take off is at the point that they get something in the $3-4K range that gets around 60 mpg, can transport two people and a few bags of groceries, and protect them from the weather. In small towns, I'm already seeing people use their golf carts to go to the store and drive around. My college runs a shuttle bus on campus, and they replaced the large van with an electric shuttle that's like a golf cart with twenty seats.