David Man's (Dennis Weaver) car: 1971 Plymouth Valiant
The Truck Driver (Cary Loftin): 1955 Peterbilt 281
Information:
In the DVD documentary, Steven Spielberg observes that the fear of the unknown is perhaps the greatest fear of all and that "Duel" plays heavily to that fear. Throughout the film, the driver of the truck remains anonymous and unseen, with the exception of two separate shots where his arm beckons Weaver to pass him and another shot where Weaver observes the driver's snakeskin boots. His motives for targeting Weaver's character are never revealed. Spielberg says that the effect of not seeing the driver makes the real villain of the film the truck itself, rather than the driver.
The truck, a 1955 Peterbilt 281, was chosen for its "face". For each shot, several people had the task to make it uglier, adding some "truck make-up". The car was also carefully chosen, a red 1971 Plymouth Valiant with an underpowered engine to signify the weakness of the David Mann character. Its red color was also intentional, for Spielberg wanted the car to appear well among all the dust and rocks on the road, which are basically brown and green. The shots of the truck are done in such a way as to make it seem "alive" in terms of its attack on Mann. According to Spielberg, the multiple license plates on the front bumper of the truck are meant to subtly suggest that it is a serial killer.
http://en.wikipedia.org/wiki/Duel_(movie)