We generally like to look at diffs independent of offroad use because T-case gearing is so much more efficient meaning you can get a LOT More reduction for your money in the T-cases. But.... (there's always a but) if it's a truck you drive on the road a lot, you definitely need to gear it to run right. 4.56 and 37s is in the range of OK with the slighly shallower overdrive of the 4L80. We've found that 4.88s/37s comes close to matching the factory 4.10 gear with the stock small tires which is "work truck" gearing more than lighter duty trucks running more highway efficient gears. I personally prefer to match "work truck" gearing since we're basically making these things drive like they have a full load of tools in the bed or a small trailer behind them all the time. And I've found that more RPM is better than less pretty much all the time.
So, for where you're at now: does it go down the highway the way you want it to? I wouldn't look at re-gearing to go to 4.88 unless it's a real problem and 5.13 is a pretty big jump. Appropriate if you think 40s are in the future but if the 4.56s do OK for you on the highway, then T-case gearing is where it's at. Doubler or Magnum. both work and have different efficiency points for different people. Somewhere in here you have to address the engine and if there are any upgrades you have in mind that will change the power curve. Obviously anything to add torque is a bonus. Recent experience with putting heads and a cam on our Wanda truck ('05 GMC2500 6.0L) indicates that you'll want to keep a cam small if you mess with it. We used a 209 intake duration and it's not bad but smaller wouldn't hurt it at all. Real point of all this is that changing power delivery can change hwy gearing needs and if the 4.56s are marginal in any way, doing some engine mods to add torque at cruising rpm might make it work.
And, Moab is pretty forgiving of and in fact often rewards higher gears so it's not really a place that a magnum in all the lows will shine. Tighter trails and/or bigger rocks change the equation a lot. The UA trip we just did in the midwest is a good example. Tight trails with lots of trees and some pretty serious rocks mixed in created a good recipe for using all the low gears. But low-low still wasn't the recipe for all of it. Gotta use the shifters to make the gearing match the problem.
Thanks Stephen, that gives me a lot to think about. I actually wasn't completely happy with the 4.56s on the highway either. On hills and steeper grades it felt like the truck was lacking power and the 4L80E would hunt between 3rd and 4th. At 80 mph I'm around 2700 RPM.
Moab was similar. In low range I felt like I still had to give it a lot of throttle and power up some of the steeper climbs instead of being able to crawl them the way I wanted. Having 4 low working on the second day made a huge difference, but I still wanted more reduction.
I'm on 37s now, but I would love to go to 40s eventually, which is part of why I was considering going all the way to 5.13 instead of 4.88.
I do have some engine upgrades planned. The engine is an LQ9 6.0 and I already have a Howards 216/224 @ .050, .551/.551, 114 LSA cam for it. After reading what you said about the 209 intake duration cam in Wanda, I'm curious what you think about that cam in a heavy truck like mine. I don't want to trade away the low and midrange torque that I need just to gain horsepower up top.
I would love to run a Doubler or Magnum. The biggest thing that has kept me looking at axle gears instead is cost. I just bought a new Tom Wood's front driveshaft, and moving the transfer case means I'm potentially back into modifying or replacing driveshafts. That's why 5.13s started looking attractive. They would help the truck everywhere and wouldn't change the current driveline layout.
My ultimate goal is to have a truck I can comfortably drive across the country, get on the freeway, climb grades without the transmission constantly hunting, and then hit the trails when I get there with enough low range that I'm not having to use a bunch of throttle to get through things. I'm not trying to build a dedicated crawler. I want a truck that does both well.
Given all of that, would you still leave the 4.56s and put the money toward a Magnum/Doubler, or does the fact that I'm not really happy with the 4.56s on the highway and may eventually run 40s change your thinking?
