You could be right, I've seen the same charts. From my personal experience, with my setup in OD @70MPH I'm hitting around 1700 RPMs which corresponds to the chart. When I take it out of OD and it down shifts, I'm at 2600 RPM @70MPH which isn't that far off from the 2380 and honestly, it just doesn't feel right to me cruising at that RPM.

Hopefully, someone who is actually running 5.18 gears with the 3.8L auto can comment. Personally, I'm leaning more towards 4.88's or 4.56's. I want to maintain some gas mileage for my daily commutes to work.

 

I understand . I'm old so I remember before everything had overdrive transmissions and I was cruising around in a Trans Am at 2500 rpm doing 60 mph! And that was with something like a 3.08 rear end!

 

Some quick math because I'm bored:

The OD ratio on the 3.8 Auto is .69
Whatever rpm you get from that chart, just divide by .69 to see where you're at with OD Off.

Ex.
3.73 with 35's (that measure 34") = 1781 rpm @ 70 (od on)
divide by .69 = 2581 @ 70 (od off)

5.38 with the same tires = 2569 @ 70 (od on)

Kind of puts things in perspective when only looking at the numbers. Personally, I like to be around 2500 rpm for normal driving. Some people like (and can get away with) lower rpm's, just depends.


For me, I wasn't happy with 3.73 and 33's. 5.13 and 35's was good (splitting time between Colorado/Texas). 5.38 and 35's was better (mostly Colorado). Now running 5.38 and 37's, (nice combo for Texas, and good for Colorado).

It's all about personal preference, your build, and where/how you drive.

 

I ran OD off for years with 35 and 37" with the 3.73. OD off is the final drive equivalent of 5.13 gears.
I now run 5.38 gears with OD On and it's not much different on the road. However, the bottom end is much better with the regear ...

 

I have ran 5:13's and 35's for 6 years. Runs great! I had ran them with 4:10s for a while... and this is much better!

5:38 might be even better, but I don't think you can or would want to get to that ratio with a dana 30... 5:13 is the max with the 30.

A wind will still force me to drop out of OD. Most hills it pulls out too. 75 is very comfortable... 85 starts feeling like you better have said your prayers...

 

Most hills pull you out of OD with 5.13's? Going how fast?

 

Definitely done this combo many times, so I can speak to it. The 3.8 likes to rev and does fine all wound up on the freeway, with 5.13s and 35s they just kinda spin and make noise, if that doesn't bother you you're good to go. Any dip in fuel economy over a 4.88 with 35s is marginal. The 3.8 really drops in fuel economy when it gets hot due to its tendency to guzzle raw fuel to cool itself... it's kind of a terrible motor, but it will get you where you need to go.

Like a poster said earlier there is definitely a strength consideration if you're wheeling it with low gears in the diffs. A Dana 30 does fine with a 4.56 gear, it's ok with a 4.88 but don't treat it like a rock bouncer. On 5.13s the teeth on the pinion get really small with small contact pattern and while some guys run them, you have to be careful and know when enough is enough.

 

Thanks for the insight.

I would love to put some D60's on my JK, but for the price I really can't justify it for the type of offroad use I do. I'm not a rock crawler. I love to play in the snow, mud, and small hill climbing. Therefore, I see myself sticking with the D30 until it blows up and I have to replace it.

I'm not planning on selling my Jeep and my long term plan is to do an LS engine swap when this 3.8L engine dies. So with that in mind, that's why I'm debating between 4.56 or 4.88 gears for this D30. I'm thinking with the combo of 4.56 gears and a LS V8, as my long term plan, that the 4.56 and that engine will give me plenty of low speed torque. However, I'm not sure which will be better for DD use with the LS engine. I'm kind of leaning towards 4.56 due to just being a little stronger than 4.88's, and 5.13's I'ld just be too concerned in breaking them on a D30.

 

75 to 80... if it is a long hill, or a "real hill"... the sort the trucks slow down to 35 on... Smaller variations in Montana are real normal, when I say "most hills"... I mean good sized hills.

I can sustain 70 to 75 in 3rd... at about 4000rpm pulling hills... but she works pretty hard!

 

I have 17 x 9 rims with 33 x 12.5 tires with 4" backspacing and the tires are flush with the outside of the fender flares.