i have a 96 f350 crew cab dually 5 speed also stock. when i first bought the truck almost 4 years ago, i was able to get 18 to 19 mpg driving normal speed limits here in jersey. i have 4:56 gears for towing. eventually my milage has dropped to approximately 10 to 11 mpg. i was told by many other shops that the problem that i have was that there is a small tube from the front of the passenger side exhaust that gets clogged with carbon that sends a signal for the exhaust to open and close properly. in the winter time my truck first starts and sounds fine out of the exhaust but then repeatedly converts to a hissing sound and this will definitely lower my fuel milage. apporx. $37 in parts but about 8 hours labor. hence, it is still not fixed, but the truck still drives normal with a lower fuel milage.
joe
