Everyone's got their own preferences about how their jeeps should drive. Keep in mind all JL power mods are on manual trans jeeps. Because it makes dyno testing far easier.Originally Posted by gbaumann
That being said, The auto is geared lower than the manual trans. So my position is to stay at 4.10 as the 37" KM2 (which i am VERY intimate with) is really a 35.5" tire. Im sure the OP had read the "truth about gears" thread, the rest of you reading this if you have gear questions, its a good reference based on my own misery.
Here's my answer. with 4.10 with the KM2 your RPM at 70 mph should be around 2288. (its the 35'' line). That is STILL more agressive than the stock rubi auto with 3.73 gearing- 2276. Chrysler offered the 3.73 gearing with the rubicon because they felt that it was the correct gearing with the new auto trans. They kept the 4.10 gearing as an option because people would have rioted in the streets if they didnt. Someone from AEV did a great article about it, I'll try to dig it up.
So lets subtract your SC. With or without the SC the 37 inch KM2 is a very light tire for its size and is not really 37". Your auto with 4.10 should have no problem turning them...happily. The SC just puts an exclamation point on it. It should spin those km2s with no problem. Hopefully your wheels are light also. Beadlocks may throw this off a bit if your trying to spin a 40lb wheel.
My point- In a case like this, my suggestion, is that you install the SC and live with it for 6 months. If your truck is a pig, (and it shouldnt be) then consider gearing. A LOT can go wrong with a gear change. you can have wrrrrrrrrrr noise, and other issues if the shop that does it, cuts a corner, or if a seal comes unseated. Snarf and I have both done gear changes and both of us have had small leaks to contend with. Just little nagging issues. Nothing terribly serious, but a pain in the butt none the less.
We all read these forums and think, the math says 37s = regear. Not so fast. The internet is flooded with old information and you need to do the math for yourself. I got caught in it, and went to 4.88s with 37s, and I wanted to kill myself with my truck spinning at 2800 rpm on the highway.
If you were at 3.21 I would say pull the trigger. I'd suggest no one for gears past Yukon, and feel that their customer service is fantastic, as they have had engineers on the phone with me trouble shooting my less than perfect install. No problems with my Yukon gears or axle kits, but the guy who put them in was a friend and not a shop, and you know how that goes.
hope this helps. Let us know what you decide.
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