Maximum Drivable Slope

Fully loaded on pavement I pulled a 200 yard hill that was about 24%. I topped out in granny low with rpm's dropping below 1000. I had to engage the power divider to prevent spinout.

Off road I have done quite a few grades over 20%. When they get icy or muddy it becomes more challenging.

 

I've taken my mixer up and down some unbelievable grades. Going down, have to go nose first, or the concrete would spill. Was on one once, wear we actually slid down, brakes fully locked, was that steep, gravity powered truck lol.
Going back up, had to go nose first, so the weight of the engine & front end keep the truck from going end over end.

 

A local quarry has a very steep road coming out of the hole. I don't know how steep it is but it's much steeper than anything you'd ever encounter out on the road. I've never hauled anything up it myself but I saw a guy in a old Ford LTL9000 haul a 100,000# crusher out of there. He was carrying the left front tire most of the way. It was a impressive sight and I was waiting for something in the driveline to give but he made it.

 

Alot of variables apply to the answer to your question.

Motor hp
Transmission type
Clutch type
Gear ratio (axles as well as transmission)
Tire types

I've seen log trucks as well as north Dakota sand and water trucks. Traverse may types of grades.

Search YouTube you may find some videos on this. Good luck