How much economy do you need? Curious what numbers your aiming at.
Jack McCornack has been getting over 80 MPG on his diesel locost that he has moved to a Lola inspired body. Picture below. He says he's working up to 100 MPG. That's pretty decent and it's more or less a real car, no roof so maybe not so great for the winter. He entered a "no petroleum" race and won it on vegatable oil
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I think that will be hard to beat with a motorcycle 250cc. Maybe not...
When I said locost chassis, I meant loosely but here's one that's done well. A lot more effort on body to look like the volswagen in your pictures would get you more mileage.
The stuff your talking about at the relatively extreme end of the spectrum could do better. I don't mind adjusting my thinking that way, but the amount of effort goes up with fancy bodywork.
I think drag is the big issue, motorcycles don't weigh much but most don't get more then 50 or 60 mpg. I've never owned a bike intended for mileage though
Before they got into downforce racecars in the sixties were designed to be low drag. So some of the old Lotus, Lola etc. were pretty slippery.
A last thought was I saw a Chinese company selling little off road vehicles that had 5 speed transaxles. JUst because your going to be economical doesn't mean you have to stop having fun!