I've gotten along with 31" tires, 6.32 and 3.54 for years, and with 2:1 in the D20. There have been a few times that I've wanted something lower but got along without it. I am going to put 3:1 gears in the case if I can keep my PTO, and that should take care of it.
The only times I've wanted something lower is in rough rock climbing, so I think it's a matter of what you like to do. But 70:1 crawl should be good for almost anything if your tires aren't too big. Don't overlook that factor.
The only times I've wanted something lower is in rough rock climbing, so I think it's a matter of what you like to do. But 70:1 crawl should be good for almost anything if your tires aren't too big. Don't overlook that factor.