[slowmover]

I seconded the "nomination" by a seasoned, well-informed and experienced Dodge CTD owner who would like to have a manual transmission in our 1T pickups which featured two overdrive ratios. The lower one for when towing, and the higher for when running the highways solo. Our trucks come stock with 3.73 rear gears, and while that man (living in mountainous Idaho) might want 4.10's, I'd want 3.42 living at sea level. Both of us have the same goals after a fashion in that this would work for us both.

Your questions are all valid, OP. But you need to be specific about what work you plan for the truck in order to come up with a reasonable plan. That does mean you'll have to narrow the range of what is possible. Climate and terrain speak directly to what is possible for a given horsepower demand (and the length of time it is called upon in full). Gearing is only half of the question as cooling system capacity is the other. Both, together, delineate what is reasonable [possible] for a given locale per work definitions.

How to specify trucks is a full-time occupation in the departments of commercial vehicle sales and manufacture. It bears no resemblance to car dealers.

The order for vehicle performance (FE among them) is:

1] Truck specification
2] Climate
3] Terrain
4] Truck use

I recommend your working them backwards to arrive at number one.

I would also recommend working from a well-known set of drivetrain choices already proven to work together. The Cummins 4BT and New Venture NV-4500 transmission are one such set (aiming at FE). Tire height and axle gearing then tend to fall into place accordingly (with top speed and weight limitations, etc, per cooling system capacity).

If you will search for discussions of the SAE J2807 Towing standard (online) of how and where testing was conducted, you'll see some of what is entailed. There are ways around this, but in main it will serve.

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