A few months back I had a 2004.5 Dodge 4x4 Cummins with the automatic...I had to trade it in a month later for the Dually..Both exactly the same , 4:10s cept the Dually and the 6 speed vs. the auto.. The automatic was getting over 16 mpg up to 1800 miles.. The 6 speed I have now gets 14.5 with 2700 miles on it.. Still, I have not seen a MPG increase...At the end of this week we are hitching up a 13k lb plus 5ver and hitting the Coast.. Hope this will help break in the engine thereby increasing fuel economy. The auto took off like a "raped ape" and the manual is "painfully slow" when leaving a stop sign.. With the 4:10s in the auto at 2000 rpms the speedo hit 67 mph.. The manual at the same rpm hits 63 mph...The auto held the trailer better on down grades and the manual "required the exhaust brake"..

 

Are you saying you traded in an auto srw for a manual dually?

If that's the case, I would think getting the same mileage would be good. The dually should get less mileage, if they are equipped the same. Unless I am missing something..

 

The dually is going to get worse mileage. That being said, if you are hammering on it and winding out the turbo before you shift, its gonna get worse mileage.
Scott

 

The AUTO final gear ratio is TALLER then the 6 speed also.The AUTO should get a hair more mileage if comparing apples to apples.