nthinuf

Just because they 'deal' with JK's all the time doesn't mean that they really have a grasp on the differences in the engine and transmissions. We don't have the old high output inline 6 or a transmission with gearing that is matched to the engine output. Look at the charts in that high altitude threads linked above and see what you come up with.

You are running 40" tires. With this engine/transmission, you need a lot deeper axle gearing than 5.38 - it just isn't available. So, you are running OD Off to keep anything close to freeway speed.
If you choose to drop down to 5.13 or 4.88 to bring your od off rpm's down at freeway speeds, will you have the hp/torque to actually hold those speeds? Or will you be dropping down to second to get your rpm's back up over 3k so you can go 70?

Tuto

I do not belief in those charts.

I would to see comments from someone that actually have the 5.38 gears and an Auto.

My 2010 Rubicon, Auto w/ 4.10 gears stock tires 32, at 70 mph is about 2300 rpms

The chart reads 2080 rpm w/ 4.10 32 tires and auto.

tslewisz

Have you measured your tires?

TexasTim

I'm not guessing I have auto trans 35s and 5:13s. It's the perfect combo.

Loadbreak

I have an Auto and the above chart was spot on when I had my 29" stock tires and 3:73 gears, then with 33"s that measured 31.5" then w/ 35" tires then the upgrade to 5:13's. It was spot on every time.

mjm

I totally hear ya and all that I have read about 5.38's makes total sence. It just gets frustrating when you go to the experts (local shops) and say I want 5.38's and they act like that it's a crazy request. You would think that the local shops would be very seasoned on the JK as it's been out since 2007.

nthinuf

As noted above, the chart is using actual tire heights. Not the mfg's spec. And not what you think your tires measure.

My AT"s, that the mfg lists as 33.8, were 32" when under the front of my jeep. My MT's, that the mfg lists at 34.8, are 33.5" mounted and sitting under my jeep.

So the moral of the story is ---> go measure...

thrshr01

I believe when you go above 5.13 and hit 5.38's the pinion gets smaller, making it weaker.

Anaya

I'm running 5:13 with 35" tires auto at 70 MPH I'm turning about 2400. I think 5:13 are better than stock I like them but I also think 5:38 would be fine you would have room to go up on tire size if u wanted.

joe002

With my auto, 5.38s and 35" BFG KM2s (which aren't really 35 inches), eyeballing the speedo and tach, and checking/adjusting with a spreadsheet...

55 - 2,000
60 - 2,190
65 - 2,370
70 - 2,550


FYI - my uncorrected eyeballs saw:

55 - 2,000
60 - 2,200
65 - 2,400
70 - 2,550