You will reduce towing capacity with the 3.31 gears and might actually hurt fuel economy as well. These engines produce peak torque at around 1600-1700 RPMs and work well with the 3.73 gears at highway speeds when towing. When not towing the 6-speed transmissions provide 2 overdrive gears that more than compensate for the lower gears.
The factor that most affects MPG is the driver. Fleet truck operators have drivers that average 6 MPG with the same rigs that other drivers manage to average 8 MPG over the course of a year. That is a 33% difference and it is widely known that there are "8 mpg drivers" on the road and it is how they drive that is very different.
I still find it difficult to understand why so many people are concerned about MPG before they buy a truck and then drive at 10-20 MPH over the speed limit and later complain about the lousy fuel economy and high price of diesel.
