In just a few words: Bullet-proof Rockcrawling. YJ-1 is being purpose built as tough, but streetable, rockcrawling machine.

When your out on the rocks slower is better. Slow gives you control over the rocks and lets you idle over things easily. To go slow you need more than just low engine RPM, you need the torque (power) to get you over the obstacles as well. Torque multiplication in the gear box is what lets you go slow enough to maintain control while still having the power to get where you need to go, like the old-timers say "It's all in the gears".

The overall measure of how slow you can go is "crawl ratio". To determine your crawl ratio you multiply your transmissions first gear ratio x the transfer case low range x R&P ratio. For project YJ1 that turned out to be:

3.73:1 x 2.72:1 x 4.10:1 for a total crawl ratio of 41.6:1.

The "holy grail" of crawl ratios is around 100:1 and we were a long way away from that & the 35" tires that YJ-1 wears makes the actual trail crawl ratio even worse than the 41.6:1 would indicate. Now we could change the R&P out (again) at both ends (for between $1,500 & $2,500). If I substituted 4.88:1 then the total crawl ration would be 49:1, but highway cruising would suffer tremendously. So for YJ-1 I decided on 2 major modifications: New transmission with a very low first gear and overdrive, and a change in the transfer case (see Atlas review).

So at this point we had decided we needed a bulletproof 5 speed transmission with a low 1st gear and an overdrive 5th gear. The NV4500 is just such as beast, it has quickly become one of the most popular manual transmission swaps for off-roaders and may be one of the best overall off-road transmissions. It has a 5.61:1 first gear (a 6.31 is available but rare) and a 0.73:1 overdrive 5th. In contrast the AX15 has a 3.73:1 first gear and a 0.79:1 overdrive 5th.

With the NV4500 installed our crawl ration would improve from 41.6:1 to 62.6:1 and that would actually result in much better performance on the trail! In addition the higher OD ratio would allow lower engine speeds on the highway and improved gas mileage (or the use of lower R&P gears without ruining highway performance).

The NV4500 is found in both late model GM & Dodge Heavy Duty trucks (GM versions of the transmission require a new input shaft). It is not strictly a bolt in installation, but a few vendors have the parts to make it much simpler. Advance Adapters supplied the transmission and all of the parts to install it in our Jeep.

If you buy the transmission & adapters from Advance Adapters you are getting a brand new factory transmission and they do virtually all of the tough work for you: all input shaft modifications are done and the necessary adapters are assembled into place. This lowers the difficulty rating for the driveway mechanic a notch or two.