This was done quite a few years ago. Take a look at this:
The California Commuter - World Record Holding 155 MPG Freeway Legal Car!
I purchased a set of his plans/booklet and hopefully will finally get around to building a modifies version this winter. My goal is to use a more upright seating position (the designer discusses this in his book), independent front suspension and a larger engine (probably a Ninja 250 engine). Probably a bit more robust frame, too.