All of the "moonrunner" cars in the film had actually been used by moonshiners in the Asheville, NC, area, where the film was shot. The moonshiners sold the cars to the film company in order to buy newer and faster cars.
The role of Robin was offered to Elvis Presley, who was very interested, but the idea was nixed by Presley's manager, "Colonel" Tom Parker, who demanded more money for Presley's services than the producers were willing to spend.
The locations heard in the song "Thunder Road" (sung by Robert Mitchum) actually describe real places in east Tennessee and Kentucky. The "Thunder Road" route runs south from Harlan, KY, thru The Cumberland Gap to Maynardville, TN, just north of Knoxville. The moonshine runner then goes through downtown Knoxville and onto Kingston Pike where he crashes on Bearden Hill (Bearden is a Knoxville suburb). The route is easily followed on a map.
The 1950 Ford that Robert Mitchum drives in the beginning is actually a 1951 Ford with a 1950 grille and the chrome windsplits removed. The giveaway: the V-8 emblems, the "Ford Custom" emblems on the front fenders, the dashboard and steering wheel.