Sorry.... didn't know you really wanted the calc. It's pretty straightforward math. Here you go:
1st Gear ratio X transfercase ratio x rear axle ratio = Crawl Ratio
For an automatic, you usually double the ratio of the 1st Gear (for a TH400 with a 2.48 gear, I use 4.96) this allows for slippage of the TC at low RPM which effectively gives you a deeper ratio (less turns of the tires for a given engine RPM)
In my example:
TH400 x Atlas4 x 4.10s
= 4.96 x 5.44 x 4.10
= 110.62:1 crawl ratio
And as you noted, that's just about perfect for a 38" tire with a 120" circumference.
I think you're talking about Mog axles when you talk of the higher pinion? The Mogs use a portal-style housing which raises the pinion relative to the wheel centerline. Very cool.... I bought a set already that I intend to use (Phase 2). Unfortunately, the axle ratio is a real ball-buster (7.56:1) for the highway it's almost unmanageable. The only solution I could come up with was to use a really deep 50% overdrive transmission (T56 from a Dodge Viper)...so I bought one of those as well (the tranny, not the car!) and have that sitting next to the Mogs for now also.
The other problem with Mogs, is that the axle tubes being so high creates a LOT of interference problems with the frame. The solution is to go full tube chassis or lift the truck really high to get the clearance needed to flex. I'm going with the full-custom chassis route......more time, more $$$. So you'll have to be a little patient if you want to see my photos of that setup.
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