Dustdevil
Member
A couple of things to consider:
Most gasoline engines in factory trim obtain maximum fuel mileage not by lugging the engine and mashing the gas pedal to get torque. That's actually about the worst thing you can do. Wrecks fuel economy AND labors the engine unnecessarily. Fuel efficiency is improved more by operating at or near (below, not above) the max torque output of the engine. Not by lowering rpm and making the engine labor. The max torque of most factory versions of the 4.0 is between 3000 and 3500 rpm. But they don't usually like to operate in that range long term. So, somewhere between 2700 and 2900 is pretty good for distance driving and highway driving. Having said that, your 6 speed NSG370 and 33" tires would yield a highway cruising speed of around 70 mph at 2750 or so rpm, IF you had 4.10:1 axle ratios. But you don't. 3.73:1 ratios are too high for 33" tires, as you will struggle to attain much above 2500 rpm at all, and especially on any sort of hill. The constant shifting will drive you nuts. That's true whether you have a NP231 (2.73:1) or NP241OR (4:1) transfer case. Either of those are 1:1 in 2WD High range, which is your highway mode.
Off Road, the 4:1 ratio of the NP241OR provides nice benefits, AS LONG AS YOU ARE NOT PLANNING A LOT OF MUD DRIVING. In mud, you often need 4WD low range to yield some wildly spinning tires without stopping to chose between 4wd Hi and 4wd Low. Mud is where the 2.73::1 ratio in 4Lo of the NP231 can be a big help. Having said that, a "doubler" or an auxiliary series gear reduction device poses several issues you might not have considered. First, it has to fit somewhere. If you mount it at the rear TC output, you affect gearing only for the rear axle, not the front axle. If you place it between the transmission and the transfer case, you alter gearing to both axles, but you relocate the transfer case rearward. That causes driveline angle issues YOU will have to solve with both driveshafts, and you'll have to replace both. Even in the LJ, there is limited room to throw driveline parts at it. My guess? You'll end up spending twice as much money and time trying to get some sort of driveline "doubler" to work than if you simply found a used NP241OR in a wrecked Jeep and freshened it up. Probably more than if you simply bought an Atlas and associated adapters.
The added complication of being the guy breaking new ground with a complicated idea can be a huge waste of time. You are the ultimate guinea pig. At work, we call that a "science project". We avoid them like the plague. Customers looking for truly unlimited free engineering help get the polite "decline to quote". Meanwhile, the guys with 4.10 or 4.56:1 axle ratios are buzzing around getting great fuel mileage without all the Rube Goldberg cutting, welding, measuring, risk, and chopping up the Jeep.
I think I would simply find a good axle gear shop and go that route first. Then consider your next move if you still want changes.
Most gasoline engines in factory trim obtain maximum fuel mileage not by lugging the engine and mashing the gas pedal to get torque. That's actually about the worst thing you can do. Wrecks fuel economy AND labors the engine unnecessarily. Fuel efficiency is improved more by operating at or near (below, not above) the max torque output of the engine. Not by lowering rpm and making the engine labor. The max torque of most factory versions of the 4.0 is between 3000 and 3500 rpm. But they don't usually like to operate in that range long term. So, somewhere between 2700 and 2900 is pretty good for distance driving and highway driving. Having said that, your 6 speed NSG370 and 33" tires would yield a highway cruising speed of around 70 mph at 2750 or so rpm, IF you had 4.10:1 axle ratios. But you don't. 3.73:1 ratios are too high for 33" tires, as you will struggle to attain much above 2500 rpm at all, and especially on any sort of hill. The constant shifting will drive you nuts. That's true whether you have a NP231 (2.73:1) or NP241OR (4:1) transfer case. Either of those are 1:1 in 2WD High range, which is your highway mode.
Off Road, the 4:1 ratio of the NP241OR provides nice benefits, AS LONG AS YOU ARE NOT PLANNING A LOT OF MUD DRIVING. In mud, you often need 4WD low range to yield some wildly spinning tires without stopping to chose between 4wd Hi and 4wd Low. Mud is where the 2.73::1 ratio in 4Lo of the NP231 can be a big help. Having said that, a "doubler" or an auxiliary series gear reduction device poses several issues you might not have considered. First, it has to fit somewhere. If you mount it at the rear TC output, you affect gearing only for the rear axle, not the front axle. If you place it between the transmission and the transfer case, you alter gearing to both axles, but you relocate the transfer case rearward. That causes driveline angle issues YOU will have to solve with both driveshafts, and you'll have to replace both. Even in the LJ, there is limited room to throw driveline parts at it. My guess? You'll end up spending twice as much money and time trying to get some sort of driveline "doubler" to work than if you simply found a used NP241OR in a wrecked Jeep and freshened it up. Probably more than if you simply bought an Atlas and associated adapters.
The added complication of being the guy breaking new ground with a complicated idea can be a huge waste of time. You are the ultimate guinea pig. At work, we call that a "science project". We avoid them like the plague. Customers looking for truly unlimited free engineering help get the polite "decline to quote". Meanwhile, the guys with 4.10 or 4.56:1 axle ratios are buzzing around getting great fuel mileage without all the Rube Goldberg cutting, welding, measuring, risk, and chopping up the Jeep.
I think I would simply find a good axle gear shop and go that route first. Then consider your next move if you still want changes.
!
