Henry Ford patented the plastic automobile referred to as the "Soybean Car”, more recently referred to as the Hemp body car. It was 30% lighter than the average car.

The finished prototype was exhibited in 1941 at the Dearborn Days festival in
Dearborn, Michigan. It was also shown at the Michigan State Fair Grounds the
same year. A patent was issued January 13, 1942 for the chassis of the soybean
car. 

Because of World War II all US automobile production was curtailed considerably,
and the plastic car experiment basically came to a halt and the prototype car was destroyed.