Gear selection for my setup

[jckid]

I'm regearing my Jeep next week. It's a 2011 2-dr. Rubicon with 4.10 gears, 3.8L engine with the anemic automatic transmission. I was never even happy with the 4.10's on the stock tires, but now I'm running 33" (285/70r17) tires and towing an M416 Replica trailer that weighs around 1,000 lbs., so it's gotten even worse. My Jeep build is lightweight (50lb. front bumper, 50lb. rear bumper, synthetic Warn winch, etc. No real heavy mods). I live in California and frequently drive a lot of mountain roads.

I've studied the gear chart over and over. I really don't plan to go bigger on the tires, so 4.88 gears seem like the best choice to me. But one shop recommended I go with 5.13's, even though I have 33's. I think their recommendation was based on my towing, which is a substantial percentage of my annual mileage (like maybe up to 50%). According to the gear chart, there'd only be 123 rpm difference between the two gear sets, which doesn't seem like it would be very noticeable, although I've read that it might be closer to 200. I thought maybe 4.88's would be stronger than 5.13's, but according to the shop there's not much difference in pinion size, since we're talking a Rubicon with 44 axles. On the flip side, one shop recommended 4.56 gears, which I think it not good advice at all since I have the weak automatic. It needs all the help it can get!

I've only found a handful of people running 5.13's on 33's. They seem to be happy, but 4.88 seems to be the more common choice. So any thoughts on the best choice for my specific setup? I've got to decide one way or the other by Monday.

 

[Septu]

I'd rather be overgeared than under geared (have been in both situations). My stock 2012 (auto/4.10s) was overgeared slightly when it was stock. Used a bit more gas, but handled the hills better. My 07 (6 spd, 3.21s, HEAVY bumper/winch/ 33" tires) was very under geared. But because it was a standard, it wasn't the end of the world. But it was a PITA.

For those reasons... I'd probably look at the 5.13s. It also allows you the option at some point to go to 35s should you choose to without the fear of being undergeared. Paying the 2k (at least that's what it would cost me) to regear my JK... is something I'd only want to do once. I'd want it to be great for my plans today, while if possible leaving the door open for future plans down the road (35s, heavier trailer, etc).

 

[camodog]

I agree with Septu. Better to be over than under, especially if you are only talking a 123rpm difference.

 

[Phoenix]

With that much towing, over rather than under.

Driving around without the trailer, you'll be running a little high on RPMs, with the trailer you will be happy.

Both my Jeeps were '12s with auto trans. Regearing the first one from 3.21 to 4.88 on 33s made a huge difference. Now on 4.1 gears with 35s and just started pulling a trailer ~2K lb when full. Thinking about moving up to 5.13.

 

[jckid]

In my research I came across the formula for calculating rpm's at a given speed. I think this might be useful. I measured the true radius of my tires and did some calculations. I usually only drive @ 65mph on the highway. With the4.88's, I'd be pulling 2298 rpm and with the 5.13's 2415 rpm. That is compared to around 1900 rpm now. When towing, I don't usually go over 60 mph, so that would be 2121 on 4.88's and 2230 on 5.13's. That assumes towing in OD. I know they say not to tow in OD, but with my trailer being so light, and assuming the new gears will be able to hold OD, I wouldn't think it would be an issue.

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So the calculations kind of steer me towards the 5.13's, along with the advice I've received here and from the shop, but my only concern is when OD is off. Now usually with OD off, I don't go over 60 mph. Currently that would be 2583 rpm, which is great. With 4.88's that would go to 3074 rpm and with 5.13's up to 3231 rpm, both of which are a little higher than I'd like. The other thing is that it seems like most people with 35" tires are really happy with 5.13's. Well since I only have 33's, running 4.88's would make me comparatively equal (rpms) to the 35"/5.13 crowd.


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So given this additional information, any more thoughts? Without knowing how it's really going to react when driving it, I still find it hard to decide. If it's going to hold overdrive on all but the steepest of hills, I might go with the 5.13's. But if it's going to kick out of overdrive fairly often, I might lean towards the 4.88's, just so the OD OFF rpms wouldn't be so high. But then it seems like 5.13's would hold OD the best. I guess I tend to over think things, but obviously I only want to do this once! :Wow1: I do plan to go out tonight and do some test runs at various rpms, so I can get the feel of where it would be which each setup.

 

[Hilldweller]

I had 4.56s on my autotranny '08 with 33s and a Conqueror trailer. Wasn't enough.
I think 4.88 will do ya. 5.13 would be more fun but you'll be spinning that thing like crazy on the highway and sucking gas like a hummingbird in summer.

It'll never plant you in your seat.

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