hey man, heres my opinion. since your off road only, go with a grsII then if you still need axle gears after that go for it. if you play in the mud, just use a higher gear. i used to run 5:12s and stock t/c, loved it, was great for lite to moderate trails. did good at tellico too considering it is a daily driver. several folks on here have wheeled with me can tell ya how it was. BUT, i have since lucked across a used grsII and got it. i ran 5:12s until i blew a rear pinion bearing (power breaking it dont help) and busted a front axle (5K rpm, dumped clutch, 2 inches of ice on top & 2.5 feet of mud underneath dont mix), i decided to install some 4:62 gears. had to keep my highway speed, is a daily driver. well, after fab fest, killed the motor a few times playing on a section of "pink" trail. pi$$ed me off, so i threw back in another set of 5:12s. so now im limited to 55 or so on the road, but i got LOW gears. if i were to start over again, i would just save my nickles n dimes n buy a grsII to start with. i know its expensive, but for my uses with my rig, i want the low gears. but, thats not to say you will need them. grsI might well be fine for you. depends on your needs. if you makin it to melt, come by, drive my p.o.s and see what its like with a II and 5:12s.
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