What gear ratio should I install in my Jeep Wrangler TJ / LJ? (and other re-gearing FAQs)
- Thread starter Chris
- Start date
It's interesting seeing the actual numbers compared to what has been bandied about for so many years. There was a time not too long ago when 4.88 was as deep as we could go. That became the ratio for 35s and all smaller tire sizes followed incrementally higher gear ratios as a result. It was wrong then and it is still wrong now.Thank you for bearing with me.
What you're missing is that the auto rubis were undergeared from the factory. Really and truly they should have come with 4.88 gears but using their mpg estimate formula, they can estimate better fuel mileage for the EPA with the higher gear ratio of 4.10. Real world that barely ever works out, because the engine is lugging so badly that it can't pull decent fuel mileage, contrary to what their estimated mileage calculator predicted.
I'm seeing that a stock auto rubi pulls about 2150 at 70 from the factory. That is way low and definitely lugging. 5.13 for 33's barely brings it up to acceptable and same for if they had come with 4.88's for the 31's @ roughly 2550 rpm at 70. Honestly if I had 31's I'd go 5.38 gears and run 70 at 2800 like most of the manuals do when they regear properly. If I had a rubi and wanted 35's, I'd gut the factory lockers and go 5.89, hands down. I would also plan for a hub kit up front because those driveshafts are spinning really fast for those ratios.
That 0.69 OD is a bitch.
Francesco.com
Member
@UKTJ I’m doing the exact same thing as you probably did, but I haven’t seen the final answer to you question so here it is: should I change the spider gears (stock 3.07) or I can keep them?
I have an 06 lj rubi auto that runs 33s for now that I installed 5.13 in. Now that I’ve had them for a while I think that 5.38 would be a great gear for up to 35. These motors are tractor motors that like between 2500 and 3000 all day long. To go to Moab I have to drive through Colorado and with the stock gears of 4.11 this Jeep was sad now with 5.13 it’s much better but 5.38 would have worked as good or better. IMHO
Help me out here for those who have the real world experience with these numbers. Currently running 32s (actual 31.5" so entered that # to the calc), 3.07gears, 4.0, AX15 5speed manual, NV231 transfer case. I like the tire size so not looking to go bigger. I'm just looking to have a better drive and not be such a slug on the road. Which would you guys recommend and why?
Screens shots from the calculator show the following results for new gears:
4.10's:
4:56:
Screens shots from the calculator show the following results for new gears:
4.10's:
4:56:
I think we have a consensus here...Thanks all. This is something I've been wanting to do for a while and this year will be the one where I pull the trigger.
I know you all are out west but any chance you or anyone in NJ knows a good shop? I'm in South Jersey but close to Philadelphia and it's suburbs. My local mechanic is a jeep guy and said he could do it but I was thinking to go with a shop that does this sort of thing all the time.
I know you all are out west but any chance you or anyone in NJ knows a good shop? I'm in South Jersey but close to Philadelphia and it's suburbs. My local mechanic is a jeep guy and said he could do it but I was thinking to go with a shop that does this sort of thing all the time.
Reidminer
TJ Enthusiast
For all highway driving? Only asking because I’m on 37’s and 4.88’s with a manual 5 speed 4.0 and in 4th going 65-70mph I’m at 3100rpms and in 5th I’m at 2600 rpms at 70mph. I find that in more hilly type highway driving I need to be in 4th to keep my speed up, as 5th seems to still dog out a bit unless it’s relatively flat
4.88 is not enough gear for 37's. The bigger the tire, the more RPMs that are needed since they not only lower engine rpms by having a larger rolling circumference, their longer radius gives the tire more leverage against the engine, and that the Jeep is further up in the windstream causing more drag. A smaller tire requires fewer rpms to turn and maintain speed than a bigger tire does at the same speed.
P.S. If you're still running a Dana 30 up front with those 37's, I recommend dropping back to 35's. The ball joints in a Dana 30 and Rubicon front Dana 44 are not big/strong enough to hold up for long with 37s.
Reidminer
TJ Enthusiast
Gotcha, thanks for the explanation. Definitely makes sense. And no, thankfully I’m not on Dana 30/35 axles. I bought my current rig when I got t-boned in December and it’s already had a lot of great work done including transplanted Dana 44’s, the front I know is a rubicon Dana 44 and the rear I believe is a Chevy 44. While I know running 37’s on 44’s isn’t optimal I don’t wheel it very hard and have plans to make big upgrades in the coming year or two. Whenever I get my axle situation right I believe 5.13 or 5.38’s would be my best bet with 37’s?
Oh good, that rear is a good solid axle. But I assure you, the ball joints in your Rubicon Dana 44 up front are not up to 37's for long. My front Rubicon Dana 44 axle has 30 spline 4340 inner/outer axle shafts, CTM u-joints, big Warn hubs, beadlock wheels, and 5.38 gearing, but I still limit my tire size to 35's. It has proven itself totally reliable with that combination.
For your 37's I'd go 5.38... that is if I can't talk you into 35's where your 4.88 gearing is ideal.
Reidminer
TJ Enthusiast
Good to know about the rear axle! And thanks for the details on your rig. That definitely helps narrow downs part choices for me haha. I actually already have warn hubs(and selectable lockers in the front axle) but I’ll definitely look into my ball joints, see what kind of condition they’re in and go from there.Oh good, that rear is a good solid axle. But I assure you, the ball joints in your Rubicon Dana 44 up front are not up to 37's for long. My front Rubicon Dana 44 axle has 30 spline 4340 inner/outer axle shafts, CTM u-joints, big Warn hubs, beadlock wheels, and 5.38 gearing, but I still limit my tire size to 35's. It has proven itself totally reliable with that combination.
For your 37's I'd go 5.38... that is if I can't talk you into 35's where your 4.88 gearing is ideal.
Stick with Spicer ball joints if you need new ball joints. So far, there are no other ball joints that will fit that surpass the Spicers in terms of quality, strength, and longevity. Don't believe the hype with Synergy ball joints either, they're a one-way trip to a set of ruined ears that hold the ball joints in place.
I would have to wonder if your speedo is correct. My lj with an auto and 33” tires only runs at 2500 to 2600 at 70. It is happy at those speeds now.
Reidminer
TJ Enthusiast
Awesome great to know! Yeah I’ve heard only good things about spicer. I’ll definitely go that route when I upgrade
Thankfully the Synergys didn't ruin my ears but they were absolutely garbage ball joints after around a year and a half. On the other hand my 20 year old Spicers were still fine and didn't even need to be replaced. Funny how we learn.
I can definitely see how they would ruin ears with the knurled body, not sure why they didn't hurt mine but I guess I was lucky. Sad that pretty much everything on the market besides Spicer is knurled, so many people can't go back.