Here's an idea that I found that's actually being produced, and comes in 50 and 150cc versions. They claim 70-80 MPG.
I'd like to do something similar, but a little longer and more streamlined. I've decided that due to it's limited use in the area that I'll eventually be living, it should be OK with a top speed of 55 MPH, and no freeway capability. I'll probably put a moped/scooter plate on it, so that anyone I know with a driver's license can operate it.
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