tufcj wroteThe t-4 will be your weak link. I like the t-18, but for highway, something with an overdrive would be nice (NV4500). But any OD trans is so long that the rear shaft on a CJ is really short, and angles way too high.
I would say that the super low gearing of the older granny low transmissions almost makes up for lack of overdrive. You can get away with stepping down your axle gears because the tranny makes up the difference.
I'm running an SM465 with 4.56 gears to push my 37's. With a 4.3 Atlas, I'm still getting almost 130:1 in low range, first gear while only pushing 2700 RPM on the freeway at 65.
To get the same 130:1 crawl ratio with the NV4500, you have to run 5.38's. With those gears, you're pushing 2400 RPM on the freeway at 65.
The freeway RPMs are only marginally better. And for the money, I like having a cast iron transmission that's basically bulletproof.
When you factor in the 401 in my CJ, the 4.56's are just fine for getting up and moving in any gear.
