Die Hard Manual ...
I was a hard core manual freaque until my '77 Blazer. It had the THD 350 or 400 and was just wonderful. I put in an after market shift kit and ran it for 14 years before selling. The only real abuse I ever gave it was the time the rubber hose to the external cooler broke at about the 13 year mark. Naturally, I was a day or so outbound and naturally I had no spare ATF. So I by passed the external cooler, dumped in a litre or two of motor oil and cursed that fact that I didn't have a temp gauge! Couple more days of heavy sand and I was back in Gaborone and flushed and refilled.
The four speed, electronically controlled slush box on Suzi the Isuzu was even better. Press the power range button and you had the perfect ticket for driving the Andes. No unwanted upshifts when cresting rolling hills.
Most of the folks who have the current Duramax/six speed Allison would have you believe that it is the cure for anything.
Notes:
-- This assumes that you know when and how to downshift the transmission without tearing it up. (Or frying your brakes.)
-- You will have to go into lower gears, sooner, with an auto trans. On the trail, I am in low range much, much sooner than with a manual, but I am in the higher gears. There is engine braking, just not as much, as fast, as with a manual.
Final drive ratio is a bit of a bear as you must factor:
-- Vehicle weight
-- Tire diameter (and width)
-- Engine power and torque
-- Goals (performance/economy)
Some examples:
-- 72 Blazer, 350 V-8, factory tire H78x15, 4spd manual (3 plus granny), final drive 3.07. Perfect. Move up to 11x15's and there is a real hole in 3rd(2nd) gear. 3.75 would have been much better.
-- I drove a 77 manual with factory 10.00x15's and 4.10 gears. A bit low, but probably spot on for 12.00x15.
-- 77 Blazer, 400 V-8, factory tire 10.00x15, auto, final drive 3.07. Good. Move up to 12.00x15 and it is still good, but again, 3.75 would have been better.
Never bothered to look with the Isuzu, but the match was perfect with the factory tires.
You can do the math, but sadly, you almost have to drive it to be sure. My instinct with larger diameter tires is to go for the lower gears. As always, YMMV.