2004 Rubicon 4.88 Gears: What Gear Oil To Use?

[Intimnasc]

That should be do-able on the Rubicon lockers. It's not impossible to break them, but should hold up assuming they're in good condition currently. The LS could definitely put you in a position to kill them later on. But it also wouldn't need the 5.13 gearing so you might be revisiting things anyways,

So actually the gearing is close between the 6L80E and the Jeep 5 speed. Ran them both on the Trenec calculator is why I landed on the 5.13's at the suggestion of another member. You can see below the results aren't far off.

5 speed manual
Engine RPM:
2500
Axle Ratio:
5.13:1
Tire Height (in):
35
Gear RatioMPH
1st Gear
4.02:1 -12.62mph
2nd Gear
2.32:1 - 21.87mph
3rd Gear
1.44:1 - 35.24mph
4th Gear
1.00:1 - 50.74mph
5th Gear
.79:1 - 64.23mph

Engine RPM:
2500
Axle Ratio:
5.13
Tire Height (in):
35
RatioMPH
1st Gear
4.027:1 - 12.6mph
2nd Gear
2.364:1 - 21.46mph
3rd Gear
1.532:1 - 33.12mph
4th Gear
1.151:1 - 44.09mph
5th Gear
.852:1 - 59.56mph
6th Gear
.667:1 - 76.08mph

 

[GoldenGorilla]

So actually the gearing is close between the 6L80E and the Jeep 5 speed. Ran them both on the Trenec calculator is why I landed on the 5.13's at the suggestion of another member. You can see below the results aren't far off.

Mathematically your planned LS will run fine with the 5.13 gears and often be closer to peak power. It may not need to be there, costing you driveability, fuel efficiency and driveline wear.

The reason that the TJ needs the lower gears is the low power of the 4.0L at low rpm. The 4.0 has a reputation as a motor that produced alot of torque at low RPMs, but it's not objectively true. At 1000rpm it's down roughly 50 ft-lbs from the current Pentastar 3.6L and the rather tame Suburban LS 5.3L is putting out nearly double the ft-lbs. At 2500rpm which is reasonably representative of the bottom end of highway cruising RPM, the LS is putting out 1.5x the horsepower and over 1.3x the torque of the 4.0L and it continues climbing as it goes higher after the TJ 4.0L torque peaks at 3200rpm.

If you peruse the JK and JL forums, people running the newer Wranglers with the 3.6 Pentastar or 2.0T motors typically do not regear to the same ratios for a given tire size as the TJ and YJ Wranglers have in the past despite the vehicles being as much as 1000lbs heavier. They don't need to because those engines produce enough power at lower RPM to move the vehicle reasonably efficiently. That's part of why the TJ needs the regear. The power to overcome aerodynamic drag plus gravity going up a hill at say 75mph combined with substantial driveline losses forces you to move up the curve which we do by lowering the final gear ratio.