There are a LOT of variables here, terrain, driving style, engine type and torque output vs. RPM range, tire size, vehicle weight, .......
In general, to compare driving an auto tranny rig with a manual rig, you're not far off calling the torque converter a 2:1 multiplier, so that evens the numbers out a bit more.
I had figured out one time that if you convert all the gearing into a standard number like inches of travel per engine revolution you could standardize a lot of the variables but I don't want to think about that right now. It's probably available for a search though.
Rigs I've played with:
My K5 with 3.73's, 33" tires, 208/700 = 29:1 and it worked pretty well for general offroad use, not much serious rock work, just general camping and trail rides.
My K5 with the same drivetrain but with 4.56's. Once again not much serious rock time but good for general use at 35.5:1 with 33's.
My K5 with a 205 instead of the 208 = 27.36:1 run with 35-36" tires. I played in Moab with it a good bit and it did OK but it was hard on the torque converter and I never got into really big slow stuff, mostly all faster moving obstacles.
My K5 with the Doubler, TH700 and 4.56 = 55:1 with up to a 37" tire. I never wanted for more gear offroad, I could have used deeper axle gears for cruising on the highway but the offroad part was covered.
My K5 with 700/Doubler and 5.13s with 42's = 62:1 I've been in a couple of isolated situations where I could use either a lower gear or a smaller cam in the motor but overall it's done. I don't always use the 4:1 low, in looser stuff I often shift up to 2:1 for more wheel spin.
My Sub with 700/241 and 4.56 with 35's = 37:1. I don't do serious boulders with it so it works just fine for general trail use and even some Moab style crawling.
Most of my manual trans driving experience is with compact trucks (S10 and Tacoma) in stock form doing relatively hard rock trails and I can tell you for sure that the stock gearing in those trucks was not OK for any kind of crawling. I got it done but the clutches smelled or we went too fast over obstacles, even given the fact that they had open diffs and I needed to move kind of fast. This opposed to my K5 with stock gearing or my 'burb with stock gearing and autos that are MUCH smoother and easier to drive.
Beck's buggy is 41:1 and miniwally is 51:1 and both do well on 42's. Beck has a harder time going slow, just as you would predict from the numbers but it's very usable.
So there's some experiences to look at.
A big factor that trusty brings to light is engine torque, his diesel has plenty of low end torque and can idle pretty slow and stay running so it's easy to get by with a bit higher crawl ratio. A toyota motor with no torque usually need at least double the crawl to get the same work done.
I've also seen differences that were less obvious to anticipate. When I first went to the Doubler, I was running a carb and it helped keep the carb happy in some situations to have the engine running more RPM since it kept the vacuum signal stronger and any sloshing fuel burned off.
Terrain? Mud and sloppy stuff typically requires more motor and wheelspin so deeper gears won't do as much good. As I mentioned before, I use 2:1 low a good bit for getting the right "bump" on ledges and for running in loose, slick terrain or really steep rock. On the same note, on big boulder trails I wouldn't make it without the 4:1. So one low gear is seldom the answer for everything.
