Kenny B

I am having a hard time deciding weather to go with 4:56's or 4:88's.
I am currently running LT28570R17's. = 32.8" and hardly ever use the overdrive.
Is any one running this set up ? I would like to know the RPM's in 4th and in over drive.
It's not really a DD but I would really like to cruise down the freeway say at 70 mph
At around 2600 in 4th maybe 2200 in o/D

2009 Rubicon 3.8 auto currently has 4:10

Looking forward to hear what you guys have to say.

Thanks......Ken

nthinuf

Click the FAQ's link stuck to the top of this modified area. Scroll down to the drivetrain section. Plug numbers into the chart. Use actual measured tire height, not mfg spec. That will give the OD ON rpm at 70 mph. To get OD Off, divide that number by .69

And it is a 4 speed automatic, so you are talking about OD ON = 4th and OD Off = 3rd? Or are you referring to the converter locking/unlocking or something?



Looking at the chart, your numbers don't make sense. A 3.8 auto, with 4.10's, running 2200 rpm at 70, would have tires that are slightly under 30" tall (again, measured, not mfg spec). And then pressing the OD Off button would bring your rpms up to roughly 3188 at 70 mph. Don't know where you got the 2600 rpm.
( 2200 / .69 = 3188 )

You are sure about running 2200 rpm at 70 in OD, and 2600 rpm at 70 with OD Off? (you should measure the tires and verify the height, and verify the rpm at 70, and verify that the speedo is recalibrated and correct for the tire height).

Donttread

There are a million threads out there that mention gear ratio and rpm

little_cornball

Please stop trolling threads and telling people with questions to do searches. I thought the forum was a community where this is what we could do to seek out help and help each other.

Chefbrujo

Lol that guy old me in my thread last night to search too. What a putz.

planman

The correct answer is "no."

With 4.88s and 33" BFGs, your engine will still run lower rpm on the highway than a stock Rubi 6 speed on stock tires.

Your jeep should have come with 4.88s with stock Rubi tires.

Your decision should be between running 5.13s to have extra power on the highway and 4.88s that put you at slightly less power than a stock Rubi 6 speed on stock tires.

arjeeper

It's a fine balance. In many cases, existing threads have much more developed information and you'd really be doing the OP a disservice by 1-2 posts of answers when multiple page discussions already exist.

Donttread

No. And to further justify, there are much more developed threads out there that would fully answer their questions if they looked. Those threads are even purposely created because it's a common question.

Chefbrujo

You could be helpful instead of not helpful. If you know of other threads either post a link or actually say what the threads you know of are titled to assist those asking questions.

Donttread

This is why the stickies exist. Did you read them or just scroll past everything?