There is not going to be a measurable difference in fuel economy but there will be a difference in towing performance with the taller gears. I never saw a difference with tow vehicles with 4-speed transmissions and now trucks have 6-speed transmissions with two overdrive gears.
It is actually silly to be even talking about MPG with a 1-ton diesel truck. The biggest single factor driving MPG is the person behind the wheel. Fleet operators know that with the same rig one driver will average 6 MPG and another will average 8 MPG or 33% more miles per gallon over the course of a year of driving.
My truck with 3.73 gears averages 14.5 MPG but gets up to 20 MPG on the open road with nothing in tow. With a heavy load and the air drag from a trailer the MPG drops to 12-13 MPG and with a 15 MPH headwind I am lucky to get 10 MPG. At 60 MPH the engine is at 1600 RPM and at the peak torque point for the engine.
Vehicles with taller gearing score higher on the EPA tests and that is all the manufacturers care about with their highly profitable trucks that are also the least fuel efficient. Anyone who still thinks that the EPA numbers are likely to be achieved in the real world needs to do a reality check.
