All the below numbers are based on the same driving conditions = No trailer and about 1000 lbs cargo, wind speed less than 10mph, summer driving conditions.
I had an 04 2500 Hemi 5 speed stick with 3.73 gears.
Highway - 19mpgUS City - 15mpgUS
My 08 2500 Hemi 6 speed stick with 3.73 gears.
Highway - 17mpgUS City - 14mpgUS
My 2010 1500 Hemi 5speed auto with 3.92 gears.
Highway - 18mpgUS City - 14mpgUS
Conclusion - I should have kept the 04.