Rockwells most common 6.72's. But can be a lot lower (like powerstoke said the 7.17 is also used, just not as common as the 6.72's due to posible breakage of the ring and pinion)
Probably at LEAST a doubled T-case, could be tripled, all with low gearing (I have seen as much as 10:1 in ONE transfer case)
Pushing those tires, he is probably close or over the 200:1 crawl ratio (I have seen Suzuki Samurai's this geared as well... come play on the rocks, we like gears !
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