I'm saying that multiple people (myself included) have grenaded transfer cases and pinion gears with similar setups to what is being discussed here. The issue isn't about RPM's nor is it about MPG.

Owners who consider 5.13 or 5.38 gearing tend to do that in conjunction with lifts and larger tires. Depending on your tire selection and your gear choice the driveline speed can be substantially higher than stock. Also your lift will cause you to make a decision on your caster angles. +4.2 degrees (stock) for longevity up to +6.0 degrees for drivability.

The 5.13 and 5.38 pinion gear is also very small in a D30 or D44. The general consensus is that under stress the housing flexes allowing the pinion gear to come out of alignment with the ring gear causing broken teeth on the ring and pinion.

Transfer case failures are also common with this setup. There seems to be the following common denominators to these failures. These are highway speeds, 5.13 to 5.38 gearing and 35" or larger tires.

Please don't misunderstand, I'm not suggesting that you or anyone else is wrong for modding their jeep. I'm saying that when we do make these choices, there are positive and negative impacts that affect the strength and longevity of our rides.

http://project-jk.com/jeep-jk-write-...-end-alignment
https://www.jk-forum.com/forums/show...+transfer+case
https://www.jk-forum.com/forums/show...=transfer+case
https://www.jk-forum.com/forums/show...=transfer+case
https://www.jk-forum.com/forums/show...r+Case+Grenade
https://www.jk-forum.com/forums/show...light=kenb1010
https://www.jk-forum.com/forums/show...light=kenb1010

 

This debate will always go on and people will go with what they feel gives them most power in the seat of the pants.

I'd like to get an idea of how many actually drive 40+ miles at 70-80mph on daily basis with 35's and 5.38's or similar.

On the trail the gears will do well since they are not really pushing the RPMs at the lower speed

However, IMO the 4 speed automatic is really a 3 speed with a very tall OD - too tall to make is useful as a true 4 speed.

The chart that gets referred to as the only thing that should be considered should have a disclaimer on it.

It does not take peak HP or Torque into consideration. With an OD at .69 the RPM fall off from 3rd to 4th (OD) is too fast and too much which is why it tends to shift out under load.

With 35" tires @70mph with 5.38's is RPM's in 4th are 2495, in 3rd it's 3616 rpm, an 1121 rpm difference!

A final OD of .78 or something actually useful as a true 4th gear would make for overall better performance. As it is now, if you run 5.38's with 35" tires or worse, 37" tires you've made OD lockout useless at higher speeds.

Theoretically 3.8 HP peaks at around 4800 RPM, Torque at 4000 RPM, if you're @ 70mph and need to drop to 3rd, 3616 RPM flattens out pretty quick since you're so close to peak torque. You'll have to slow down.

OD gear in the auto was not intended to be a true 4th gear, but an attempted solution to try and get more mileage under ideal conditions.

Run what you like, but the chart should have a permanent disclaimer on it.

I doubt many newbies read that it was created by a fellow member to be used as a guide and that the recommended RPM's are based on the OD gear of .69

I'd rather calculate my powerband range based on 3rd and let 4th be what it's supposed to be, OD - that way I still have a fully useable range of gears and since a lot of my driving is freeway,
I'm able cruise at 70+ in OD, get decent mileage and be able to lockout OD as needed and still maintain speed.

I've been watching this exchange and well, my 2cents