100mpg isnt too hard with a purpose built vehicle , and this one shows that its doent have to be entirely impractical to use either.
My favorite of all the microcars is the HMV Freeway and ime debating about making a revised version myself with much more modern mechanicals.(either that or a small speedboat).
The HMV Freeway made production in limited amounts in the early 80's , but it was plagued with setbacks rite fromt the start.
Its story ..
http://clubs.hemmings.com/clubsites/hmvfreeway/1975.htm
The car ...
Available in 1 or 2 seater in-line formats it was powered by nothing much more than a 15hp lawnmower quality engine , I think similar to what my Bobcat tractor has.
It had a simple tube frame , fiberglass body , and the rest is how you see it.