Tire sizes, gearing and axles

[murray-meg]

Just bought our first jeep in August. 2011 Rubicon Unlimited with factory 4" Mopar lift and 35" BFG KM2's on Mickey Thompson classics (non-beadlock type). Just bought bushwacker flat fenders, installed rears Sunday and plan to install fronts next Sunday. We can see now with the fenders that 37's would be the ideal match for the Rubi at least visually.

We took a roadtrip prior to the lift and tires and felt the highway performance was very good considering some of the negative things we had read. We traveled 80 mph no problem with gearing and power very smooth. After the lift, wheels and tires came in we installed an Airaid cai and a Banks catback then took another roadtrip and the highway performance was horrible; no power, lost overdrive. Initial offroad performance seems fine with improvement in low end power due to intake and exhaust upgrades.

Now for a few questions;

1. For 35's which gear ratio would be recommended for best overall performance. We don't want to sacrifice offroad performance but we also want to be able to drive on the highway loaded up with all our gear and still be able to travel at 75mph @ 2600rpm's (is this realistic).

2. That said, we would also like to consider the 37's now but wonder if there are too many consequences and upgrades required to run them. Are you able to use the stock axles without beefing them up other than changing gear ratio, and will the 4" lift be able to handle them. What other problems might we run into.

3. Do we need a performance chip and if so which models.

Any and all of your comments/recommendations will be greatly appreciated.

 

[wayoflife]

1. For 35's which gear ratio would be recommended for best overall performance. We don't want to sacrifice offroad performance but we also want to be able to drive on the highway loaded up with all our gear and still be able to travel at 75mph @ 2600rpm's (is this realistic).

if you were to stick with 35's and want to keep your rpm's down on the highway, i would say you should stick with 4.88 gears. 5.13's will definitely give you a boost in power but at 75 mph, you'd be running more like 3200 rpm.

2. That said, we would also like to consider the 37's now but wonder if there are too many consequences and upgrades required to run them. Are you able to use the stock axles without beefing them up other than changing gear ratio, and will the 4" lift be able to handle them. What other problems might we run into.

now, if you really are thinking about 37's, 5.13's would be a better match. running 37's on factory 44 axles is fine around town and for light wheeling but, if you want to start playing harder, you'll want to run chromoly shafts up front and, if you haven't done so already, have a set of c-gussets installed. sleeves are nice cheap insurance but, i would get shafts before them. the rear 44 axle is actually pretty tough and i'd be comfortable leaving well enough alone or, at least until something gives.

3. Do we need a performance chip and if so which models.

we run a superchips and feel that it's made a significant difference in performance. i would highly recommend one.

 

[murray-meg]

Thanks Eddie,

Ultimately I think we would like to see the 37's so we will start looking into the changes you suggest. We will start with gears, shafts, c-gussets and superchips. I think we will have to farm out the gears to a shop (will try the dealer to start in hopes of them standing behind the warranty-good luck right). We might like to try changing out the shafts ourselves as long as it's not too technically difficult (suggestions and opinions on experience and tools necessary would be great). Do we want to install better seals to help keep out water, dirt, etc. Any recommendations there. Also wonder if the c-gussets are something a do-it-yourselfer might attempt or do you really have to tear down everything in order to weld them on.

Thanks again and I'm sure we will have more questions as we move forward.

Murray & Meg

 

[wayoflife]

Thanks Eddie,

Ultimately I think we would like to see the 37's so we will start looking into the changes you suggest. We will start with gears, shafts, c-gussets and superchips. I think we will have to farm out the gears to a shop (will try the dealer to start in hopes of them standing behind the warranty-good luck right). We might like to try changing out the shafts ourselves as long as it's not too technically difficult (suggestions and opinions on experience and tools necessary would be great). Do we want to install better seals to help keep out water, dirt, etc. Any recommendations there. Also wonder if the c-gussets are something a do-it-yourselfer might attempt or do you really have to tear down everything in order to weld them on.

Thanks again and I'm sure we will have more questions as we move forward.

Murray & Meg

you really do want a shop familiar with installing gears on a jk to install yours. it's no easy task and there are a lot of ways to do it wrong. it ain't cheap but, well worth it do have it done right. out here in socal, off road evolution offers a special of $999 including gears. regarding your drive shaft, if you do the install yourself, do it before you get your new gears. it is difficult to install pinion yokes without compressing the crush sleeves more. when you get new gears, the crush sleeve will be replaced and so, there's no foul there.

as far as gussets go, you can do it yourself but, you really need to take it slow and make sure to cool off the ball joints on a regular basis. failure to do this will cause the plastic components inside the joints to melt.

 

[murray-meg]

Thanks again Eddie,

Seems like the shop we are looking at knows what they are doing. They specialize in Jeeps, although it appears that they really love hardcore crawling so the general tendency with them is to build accordingly. I spent a good amount of time today going over options and more or less was deterred from running 37's on the 44 axles. How it was explained to me was that with the larger tires, things tend to break more frequently, especially if you play harder.

The recommendation was to stay with the 35's and install new gears (4.88's), new Superior chrome moly needle bearing u-joints at front axles, new front and rear Superior evolution series chrome moly axle kits and Superior axle seal savers for front axles. Parts were around $3000.00, labor was about $1000.00 plus tax of course and most likely new diff covers. All said and done it would run about $4500.00. Wish we were planning a trip to socal to take advantage of the 1k gear deal.

We are planning another trip next month and we will be driving through high mountain areas to get there and really want to have the gears taken care of by then. Weren't looking to replace all of these parts but I guess since the labor is only 1k to do it all it sort of makes sense to do it all now instead of spreading it out and paying 1k for labor again if and when we break the u-joints or an axle. What do you think about that reasoning. It's just not in the budget to drop new Dynatrac 60 housings right now.

Thanks,

Murray & Meg

 

[wayoflife]

being that you have a rubicon, if you stick with 35's, i would hold off on the axle shafts as they really aren't needed. do get your axle c's gussetted up though as they will bend if you don't.

 

[TeamFaust]

I realize that this thread is cold however appreciate the information as I'm in a similar position, althought with a lower starting point in our 2008 Sahara (I really should have held out for a Rubicon). Wanting to get 35's but concerned about the D30 front axle and the gearing (have the auto tranny). One of the local guys suggested the 5:13 gears but that sounded a bit tall.

I'm really considering trying to find a SoCal JK Member (or oher source) that has upgraded their Rubi to D60s and would like to clear the locker D44's out of their garage :yup:

Sounds like the simple task of upgrading to the needed 35's is gonna be a lot more involved and spendy that just picking up new tires.:crazyeyes:

If anyone happens to have any comments/suggestions/insights (or spare front D44 they don't need) I'm listening. Thanks.

 

[JandSJK]

5.13s

I know this responce is a bit dated but i'm running 5.13s and 35's with an auto and it is to low geared for alot of freeway driving. I'm at about 2750rpms at 70mph. I also have a D30 front end had the c's gussetted and the housing sleved when i had the gears installed. Because of the high hiway rpms i'm looking at 37 when the time comes for new tires. i know there are things that are going to need upgraded when i do this but since i dont plan on running a locker up front and i dont do any hardcore wheeling i'm willing to live with the D30 and will upgrade parts as needed. I know this is not the way most people think about the D30 but i have used and abused several D30's over the years and will have to say they are stronger than people give them credit for(IMO).

 

[highoctane]

I'm on 37" MTR K's with 5.13 gears and a 6 speed. At 70 mph I'm at about 2700 rpm and think its perfect. I gained 2mpg back compared to 4.10 gears and 37s. I rarely have to drop to 5th gear, even at 6k ft elevation in the mountains. Below 2500 rpm I don't think I'd like it on the highway. But if you live in a relatively flat area, 2500 rpm or slightly less would prob work great for hwy driving on 37" tires.

 

[TeamFaust]

I know this responce is a bit dated but i'm running 5.13s and 35's with an auto and it is to low geared for alot of freeway driving. I'm at about 2750rpms at 70mph. I also have a D30 front end had the c's gussetted and the housing sleved when i had the gears installed. Because of the high hiway rpms i'm looking at 37 when the time comes for new tires. i know there are things that are going to need upgraded when i do this but since i dont plan on running a locker up front and i dont do any hardcore wheeling i'm willing to live with the D30 and will upgrade parts as needed. I know this is not the way most people think about the D30 but i have used and abused several D30's over the years and will have to say they are stronger than people give them credit for(IMO).

Interesting comments on the D30...Maybe I'll rethink a bit. As likely not going to to go to 37" tires in the near future (35's should be plenty for what I need for now). Maybe regear the current axles D30 front & D44 rear to 5:13 and beef up the D30 a bit. Still keep hoping to get ahold of a set of D44 Rubicon axles that someone needs to off-load because they upgraded...
...riiiight :thumb: