Robert Q. Riley Enterprises, a product design and development consultancy from Arizona, offer a range of their (mostly) vehicle designs as downloads, some free of charge.
image © Robert Q. Riley Enterprises
The latest is the XR3, a three-wheel sports car that can be "outfitted with different power trains to create a 125-mpg, diesel-only vehicle; a 100-mile-range, battery-only vehicle; or a hybrid for increased performance and fuel versatility." For just $170 you get "eighteen D-size (24 x 36 inch) drawing sheets, one 32 x 64 inch full-size template sheet, and a 149-page construction/technical manual with over 200 photos and illustrations." Not bad.
I've been having trouble getting the free designs to download, but what does work is a whole host of how-to information and technical papers that are also available for free. Evrything from One-Off Construction Using Fiberglass Over Urethane Foam to Three Wheel Cars: The Factors That Determine Handling and Rollover Characteristics.
An impressive site, a little hard to navigate but only for the abundance of material held within..