I geared 5.13 for 40s. And I pretty much rock crawl lite with the Power Wagon so I want the torque and reduced stress to the driveline. Fuel consumption has not changed from 12 smiles per gallon (reprogrammed ECU/TCU).
My theory for running the larger tires is to reduce stress to core components such as transmission and transfer case. Thick steel like the ring gear and axle shafts can take it, and are far cheaper than driveline components to replace. The engine and trans has to work harder to turn those big tires on the rocks and up inclines. Just doesn't sit well with me personally to have essentially highway gearing when I bought this to crawl.
That said higher gearing for cruising down the interstate to your destination fast is something important too. To me, a little more time isn't a bother.
Your 68RFE Trans can handle a good amount of torque, and with the tcase I guess the high gearing isn't as much of a problem (I wish we had the option of 4:1 factory tcases. C'on Borg Warner!)
My 66RFE tcase is a mushy ticking time bomb waiting to strand me somewhere.
Do you have a rear locker?
I'm not a fan of Detroit lockers for this type of rig. For normal driving you want an open diff so the outside tire in a turn spins fast than the inside, reducing stress to the components and wear on the tire. With a Detroit (or grizzly or lunchbox) locker your outside tire can engage on those corners causing chirping and increased wear, add ice or water and it can send you spinning.
They are far cheaper than selectable lockers. But just understand the trade off. I drove to high-school with a welded spool so I'm the last one to lecture... lockers are better than open diffs for the trail, and I'm getting the idea you like to go on aggressive trails.
Also I think you should be 30 spline axle shafts for your pricing research. If you regear yourself let me know. It's kinda annoying, there is a carrier bearing where the Tru-Lok hub access port is, so you need a long thin piece of wood or malleable material to slide between the axle shafts and tube at the hub (hub obviously removed) and knock it back into the access port. Then you can remove the axle shaft. Good to know for the trail too since it's kinds hidden. If you need to rip out the axle shaft that is. It's not easy.
I completely agree with your theory for reducing stress on core components! It makes perfect sense with all the added rotational mass the drive train has to move that gearing down will definitely reduce the stress on the trans and T-case… I will keep that in mind for sure!!!
What kind of reprograming did you do to your ECU/TCU to keep your MPG at 12??? I went from 17MPG to an average of about 11.3 to 12.5 just from putting the 37’s on… I elected to go with the 12.5’s because I knew 13 or 14 would of put me right into the single digit MPG range, and there is so much more I’m doing that is going to bring the mpg down that I didn’t want to kill it from the jump… I can squeeze out 13’s if I drive like I’m an old man though….lol
So I called Dodge… I guess my rig has an open diff in the front, but does have a limited slip in the rear… The guy on the phone said the clutch plates are probably warn, and that would be covered by the warranty….. But…, of course any locker installed will VOID my lifetime warranty on the trans and motor… So that Blows!!!! And yes…. I do like aggressive trails, but I like the slow more Technical like rock crawl trails. I don’t want to go crazy though…lol
I forgot to ask them about regearing, but I have an apt. next week for them to diagnose the rear diff. I will ask them then…
I’m starting to get discouraged about my build, but I’m going to pick the hell out of their brains next week, and get in writing the full extent to which I can modify my baby without voiding the warranty, I’m pretty sure I’m going to be severely limited to like only a 3” lift, shocks, and just cosmetic upgrades…
Sooooo I have a feeling that there will come a time in the VERY near future when I say F**k It.!!!
