So the automatic is essentially capable of a deeper reduction than it's lowest ratio when starting from a stop?
It probably wouldn't be a fun project. I'm just trying to determine if it's even feasible at the moment.
My old C65 Chevy had two speed rear (Eaton), Can't remember low gear ratio, but very low, 8:1? & 5 speed (Eaton) 427 gas tall block GM motor.
New Top Kick, very similar size. Towing a backhoe up the paved hill on Danby Mountain Road parts are 15% grade.
Old truck would run out of gas in the carburetor if the tank wasn't full, until I found a cracked O ring on a fuel filter cannister. When it WAS running right it'd be a high stress situation. Low/low full throttle, I wasn't sure if it was going to make it. More than once I'd unload at the bottom of the hill, & drive the backhoe to the top. Being near the top is a frightening situation when the truck won't pull it!
"New" truck is the same size, 500 LB more GVWR, empty truck 300 LBS heavier. Turbo diesel Cat 3208 10.4 liter is rated same HP as the 427 in the old truck. Axle is 4.99 approx. Allison crawls up slowly, but without effort. Only concern is traction, as The Allison goes to low gear & the torque converter then makes up the difference. My engine has most torque under 2000 RPM, so I keep engine at 1800-2000 & it does the rest.
I agree, some situations a manual transmission is better, & I love going through the gears, it's fun! In truth Allison is a great transmission & nothing climbs like an Allison. Allison demonstrates their ability to start halfway up a steep hill. Quite remarkable how it can feed just over zero speed to the wheels to get started without the sudden surge of power that'll break traction.