Rear ratios

That’s fine.....kinda old school. But the new way of thinking is 2.xx rear gears.

P.S. New isn’t always better

 

And I believe the closer you get to a 1:1 final drive ratio, the more efficient that gear mesh is. Where are the big fuel milage gurus at? @Ridgeline

 

A manual Transmission has to have at least one countershaft . All heavy duty transmissions are at least twin countershafts . Mack used to make triple countershaft transmission.

 

That may be. But from what I understand, a direct drive 10 speed runs the power straight through the transmission with no gear meshes, hence the greatest efficiency.

 

There is a world of difference between direct trans and OD trans.
I've ran 80k in the Eastern parts of I 80 with a direct 10 speed and 2.64, or 2.36 rear gears, it had a DD13 445hp and that thing pulled great all day. But was slow to accelerate but not by a lot.
I drove a Columbia with a OD 10 speed and 3.55 axles that thing would pass guys going uphill sometime, and would top out at 96mph that was a 13.7 series 60 at 80k most days.
I drove a crib truck with a N14 red top and that had a 13 OD and grossed 130k to 160k I'm not sure what axle gears were but I'd guess 4.55 or near that thing screamed 1800rpm and 68mph. I do know fuel avg was 4.6 mpg round trip.