The Surviving Duel Truck
There are a lot of misconceptions about the Duel Truck or trucks.  The surviving
truck is what we refer to as Truck #2.  There are 4 Duel trucks, depending on what
you count, but in an effort to be "all inclusive", the four trucks are:

#1  The 1956 Pete 281 that was used for most of the movie had a 1674 270hp I-6
Cat.  As those didn't come out until the mid 60's, that means it was a repower.  
That also is the reason for the Cat horizontal air cleaner form a 966 Wheel Loader.
 The truck had a 10 speed transmission, most likely a Fuller RT-9510.  The truck
was on its last legs during filming, and thus Speilberg ordered a second truck to be
made to look like it, thus truck #2.

#2  My 1960 Pete 281 was the second truck made as a double.  At the time
George Sack built it up, it had a 262 Cummins and a 5x3 Spicer/Brownie.  This
truck lost its Duel makeup to appear in a Malibu commercial, David Lee Roth's
"She's My Machine" video.  It was re-adorned in Duel garb to have a cameo in the
2003 biker movie "Torque".  Neil Losasso bought the truck from George Sack in
2004 and I bought the truck from Neil in October 2005.  I've had a lot of naysayers
about this truck, but rest assured I have the paperwork trail back to Universal
Studios and George Sack in 1971.

#3  The 1964 Peterbilt 351 that was used to film the extended gas
station/laundromat scene, the school bus push and the railroad crossing push.  As
was noted in a previous post, it was made up for the movie, returned to Sack's lot
and destroyed in some other production.

#4  (if you call it #4) was the mid 60's short wheelbase 351 that was used during
the Incredible Hulk episode "Never Give a Trucker an Even Break" from the first
season.  They used volumes of footage and sounds from Duel, but used this
fourth truck for new scenes with the actors.   The truck's origins and wherabouts
are unknown.

That's the quick skinny on the trucks.  Duel, like Convoy, is filled with Foley
overdub of the truck sounds.  As with most any movie, the sounds you hear are
not being captured as the video is shot, they are added during post-production.  In
Duel, you hear mostly the sounds from a Cummins and a 13 speed.