Factual error: There is a major error in the scene where Geena Davis steals the tractor trailer. They show a closeup of the brake line between the truck and the trailer being cut. The problem arises when fluid starts pumping out of the line and Geena screams because she's lost the brakes. First, every tractor trailer is equipped with air brakes, so there is no fluid in the lines to leak. Second, if these air lines are severed it causes the brakes on the trailer to lock up and drag the truck to a stop.
Factual error: The police in the scene where Charlie is asking for help are very close to her. They also know that the truck is a bomb. They would not really be that close with the knowledge they had.
Answer: A 1966 Ford Galaxie-500 XL. There's a website that lists the vehicles used in the movie.