There is an important factor that has (as far as I have read) not been mentioned that affects the choice of whether to slow the engine down for gas mileage, wear, noise etc. while cruising to achieve the same engine RPM at 60 mph, etc. by using:
A. a low numerical axle (3.38:1, etc.) with a standard transmission (1:1 final drive ratio), or
B. a higher number axle (4.10:1, etc.) with an overdrive transmission
The engine will run the same with either.
However: the driveshaft speed always follows the axle ratio (not the final drive ratio), and will be turning much faster with an O/D + high numerical axle than with a standard transmission and low number axle.
This is not important for mild cruising, but definitely affects top speed especially with long driveshafts.
If you have a long wheelbase truck (= long driveshaft), or a substantial pinion angle misalignment (transmission swap, chassis lowering, etc.) this should be examined before making the choice.
