Many people don’t realize that, although the truck attack scene appears to be shot in one day, there are many days of work involved. The trucks were finicky at best, and one was replaced with another for various reasons throughout the scene. You can determine the different trucks in the chase scenes by several key points.
Don't let the differences in color fool you, transfers, screen captures and web postings provide ample opportunities for adjusting the original colors of the trucks.
truck #1
For the sake of distinguishing between the three trucks, let’s call the six cylinder truck (my truck) #1, the eight cylinder truck #2, and the non-running truck #3. 
truck #1 left, truck #2 right
The original six cylinder engine of truck #2 was replaced by a larger eight cylinder engine which took up much more room in the cab. Since the radiator and firewall were removed during the process and the back doors were removed to improve air circulation in the cab, daylight can be see through the grill in the lower angle shots in the film.
Note the small spot of daylight in the center of
horizontal slats of truck #2. This shows up better in the film.
One of the horizontal grill pieces is rusted through on truck #2 and vibrates considerably in several shots in the film. The grill is still intact and does not vibrate on truck #1.
solid grill on truck #1
rust through on horizontal slat on truck #2
As seen in the close-up on the wheel as the truck roars down the road, the running board on truck #2 is rusted away from the fender.
The running board on truck #1 is still intact.
truck #2
complete running board on truck #1
The driver’s side front fender is dented on both trucks #1 & #2. The noticeable difference is that truck #2 has much more of a horizontal crease in the dent.
defined fender crease on truck #2
less defined dent on truck #1 Truck #3 was never complete; some front end pieces were missing.
Truck #3 is featured in the scene at the church where the Creeper is throwing a body down the pipe. The front end is obscured by a bush to hide the missing parts.
truck #3 at the church
Truck #1 is used to pass Darry and Trish as
they walk into the gas station / diner.
