A polythene car?

Erstwhile Formula 1 design genius Gordon Murray, has been working on an environmentally-friendly road car, called the T25. It will be smaller, lighter, more powerful, cheaper, more fuel-efficient, and emit less carbon dioxide, than its current main competitor, the Smart ForTwo.
In the September issue of Motorsport Magazine, Murray reveals that the bodywork of the T25 will be made from a material never used before in the car industry, but hints that "it's been staring us in the face for years. Think water bottles."

Most water bottles are made from polyethylene terephthalate (PET), so I'd infer from this that the bodywork of the T25 will be made from some type of polyethylene. Now, PET is a type of polyester, and glass-reinforced plastic (aka 'glassfibre' or 'fibreglass'), a composite material consisting of glass fibres embedded in a polyester resin, has previously been used by the car industry for bodywork. Perhaps, then, the T25 will be made from non-reinforced polyethylene. Despite this, Murray predicts a four-star Euro NCAP rating for its crash-worthiness.

Best of all, Murray points out that "the T25 weighs just 500kg - believe me, it will be fun to drive."