[06jeeplj]

I’m trying to decide what rear axle and gear ratio to get. I would like to run 42s was thinking of a sterling 10.5 but only can get gear up to 5.13. So the question is will 5.13 be low enough or go with a 14bolt or a sterling 10.25. I already have a front D60 out of a 08 ford. My jeep is the stock 4.0 with a 6speed.

 

[dustin_the_wind]

Grimmjeeper.com/gears

Enter your setup and will tell you what speed and at what RPM.
5.13s will probably be decent in low lock if I were to guess.

Sent from my Pixel 6 Pro using Tapatalk

 

[Beat95YJ]

Moving to non hardcore. You will probably want the deepest gears you can get w/ 42’s. Use the calculator.

 

[RANGERROD]

You can run 5.38’s from 10.25 in your 10.5. Plenty of threads on here about it. With a 4.0 that may not be enough gear still. You can get 6.17’s, 7.17’s for Dana 70 and low pinion Dana 60. Hard to find a driver drop low pinion front 60 though.

 

[total newb]

I helped my buddy pick a similar setup and 5.38 and 4.0l worked out well for street driving. Idk about hiway though.

 

[t_p_lee05]

I ran 5.13s in my '89 GMC Jimmy (fullsize, older 'square' body-style for those not aware) with 38.5" Super Swamper TSL's and a bone stock 350 TBI engine. I went with 5.13s then specifically because I planned to run 42's when the 38's wore out (I ultimately sold it before that happened though). This gear ratio was a little short for highway driving, but suited the anemic powertrain for around-town and off-road driving. Also was not resigned to using 4-lo as soon as I hit the dirt.

As for how this all relates to your question, I 'think' 42s and 5.13s would be OK with a 4.0, but you are sinking a lot of cash into a platform that has no headroom for going with a shorter gear down the road. Also, not sure about the Sterling, but the 14b FF is the easiest rear end to setup next to the Ford 9" with a dropout 3rd member. Also something worth considering, IMO.

For another aside, I'm running 4.88s and 35s in my 3.8L powered JK. I will be going to 37's once I wear out the 35s, but suspect that I'm going to have to reach for 5.13s given how underpowered my 3.8L is...which, nominally, has more HP and torque than your 4.0

One more tidbit...RPMs also don't tell the whole story. Meaning, just trying to match final drive ratio to that of your current or near stock setup may not net you the desired results. 42" tire are dramatically heavier and will sap power out of proportion to it's weight due to the fact that it's "rotating" weight and not 'sprung' weight. Meaning, the power needed to spin a 100 lb wheel/tire and propel a 4,000 lb vehicle forward is greater than the power needed to do the same with a 50 lb wheel/tire and a 4,050 lb vehicle. In essence, you'll want the extra get up and go provided by a lower-than-stock final drive ratio.

 

[H8DWINGS]

Is it a trail only rig? If you’re trailering it everywhere and want deeper here’s could getting an atlas at 5:1 or a 4 speed at 10.3:1 get you what you’re looking for without going 5:38s?

 

[06jeeplj]

Is it a trail only rig? If you’re trailering it everywhere and want deeper here’s could getting an atlas at 5:1 or a 4 speed at 10.3:1 get you what you’re looking for without going 5:38s?

Yeah I trailer it about 90% of the time. I’m planning on eventually getting a 4 speed atlas but right now I’m looking for gears.

 

[Mohadeab]

I’m trying to decide what rear axle and gear ratio to get. I would like to run 42s was thinking of a sterling 10.5 but only can get gear up to 5.13. So the question is will 5.13 be low enough or go with a 14bolt or a sterling 10.25. I already have a front D60 out of a 08 ford. My jeep is the stock 4.0 with a 6speed.

5.27 9 inch?