I found this old thread while trying to look up the weight of a go kart, and possible street conversions. I'm still looking for recent examples.
However, germane to the OP's question, I have a strong interest in cars that weigh less than they carry. I built a velomobile that successfully integrated the suspension into the frame
https://www.compositesworld.com/colu...mobile-history and then got interested in doing one with an engine to get 400 MPG. This is not likely to happen without a building partner who is also expert at getting permits to use it. There's a minimum seat height for trikes here.
Anyway, I have a dandy engine if someone can put it to good use. It is from a Honda Quad, with 125 CC horizontal cylinder, reverse, and electric start. 75 lbs.