Lowell Overly in the Soybean Car,1941
Weighed about 1000 pounds (450kg) less than steel Ford.
Update from a Henry Ford Museum site:
The exact ingredients of the plastic panels are unknown because no record of the formula exists today. One article claims that they were made from a chemical formula that, among many other ingredients, included soybeans, wheat, hemp, flax and ramie; while the man who was instrumental in creating the car, Lowell E. Overly, claims it was "…soybean fiber in a phenolic resin with formaldehyde used in the impregnation"