Triple checking gearing

[zonearc]

I have a 392 on 37s ... stock gearing is 3.73 with 33s. Using calculators it looks like on 37s I would need 4.10s to be back at 70mph and 1750rpms which is similar to stock, right?
Shops keep saying 4.88s and the math isn't adding up.
8th gear ratio is .67 I believe if that helps.

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[AcesandEights]

You're probably right, BUT, the taller tires are heavier and will be harder to turn. Lower (numerically higher) gearing provides more torque. So, to get you to an RPM, yeah, you're probably right. To more efficiently turn those obnoxiously tall tires, you should probably go lower.

33 to 37 is about 10%
3.73 to 4.11 is about 10%

 

[zonearc]

So, 4.10 is mathematically correct, but 4.56 would get me a bit additional torque.

 

[JT1]

I have a 392 on 37s ... stock gearing is 3.73 with 33s. Using calculators it looks like on 37s I would need 4.10s to be back at 70mph and 1750rpms which is similar to stock, right?
Shops keep saying 4.88s and the math isn't adding up.
8th gear ratio is .67 I believe if that helps.

That's a ton of money for not much difference.. IF you are going to do it, do 4.30 or 4.56.
Factory 392 XRs are 35's and 4.56s. 4.56s gets you a tick under 1900 at 70 in 8th. Extra RPM will help offset the weight, aero and gearing difference vs the 33s.

 

[grimmjeeper]

Some shops have a one answer fits all Jeeps attitude.

Honestly, if they don't understand why the 392 doesn't need the same gears as a 3.6 or 2.0, you should find another shop.

Yes, the 4.10 gets you closer to stock. But it's a lot of money for a small change and generally not worth it.

The real question to ask is whether you're talking a gear change because someone is telling you to or if you want the jeep to perform differently.

Once you figure that out, then you pick the gears to solve the problem you think you have.

For me, 4.56 would be the direction I would go if I needed to change gears. But only if the Jeep doesn't do what I want it to.

My diesel runs fine with 3.73s and 37s. So I didn't change the gears. Your 392 has a different torque curve so your situation is different.

 

[zonearc]

I picked up a used 392 and the previous owner put in 5.13s. So, let's just say that I'm trying to solve a problem (2500rpm at 70mph).

 

[grimmjeeper]

I picked up a used 392 and the previous owner put in 5.13s. So, let's just say that I'm trying to solve a problem (2500rpm at 70mph).

Yeah, at that point 4.56 or 4.10 would be good choices. 4.88 is too small a change from 5.13 to be worth it.

 

[JT1]

I picked up a used 392 and the previous owner put in 5.13s. So, let's just say that I'm trying to solve a problem (2500rpm at 70mph).

That's a ton of gear. Bolt some 39s on it or drop them to 4.56s

 

[zonearc]

It had 40s on stock axles on 3.5" lift, so it was rubbing and it was risky. I've since swapped the 40s for 37s, put in RCVs, Dynatrac ball joints, trussed the axles, Delta brakes, bumpstopped it properly, etc. The idea is to "built it right" for what I want and I don't need yet another jeep on tons on 40s ( I have those already). So, now I need to tame down the gearing. 4.10s sound good on paper, but I get everyone's point on compensating for weight and the vehicle being shaped like a brick so I'll do 4.56s.

Thank you all!

 

[AcesandEights]

I feel stupid, but I didn't realize you were going down in gearing. How about 4.30 gearing? I think those are made for the D44, but don't quote me on it. Also, depends on how you use your rig.

 

[2nd 392]

After 35’s the gearing charts showed 4.10’s / 36’s equal to 3.73’s / 33’s. .Yet the XR’s 0-60 & 1/4 mi times were just slightly better so the chart doesn’t consider weight and rotational mass. 4.56’s are the sweet spot for 35’s IMO but would certainly pull 37’s fine as 3.73’s pulled the 35’s fine, but 4.88 likely the sweet spot for performance without excessive hwy rpm

 

[AnnDee4444]

I picked up a used 392 and the previous owner put in 5.13s.

Dang... 4WD burnouts?

Here's the RPM @ 70MPH in 8th gear

 

[zonearc]

 

[autotragic]

I know the "hive mind" says 4.88/5.13 but 4.56 is more than enough for me after doing the math and actually looking at the data. In fact I have determined that 4.10s are enough for 37s for me personally. Only way I'd go 4.56 is for 38s or larger.

 

[JT1]

I know the "hive mind" says 4.88/5.13 but 4.56 is more than enough for me after doing the math and actually looking at the data.

4.88 and 5.13 are for the 3.6. The 2.0 needs less gear than the 3.6. The 392 is in a world of it's own.
The diesel doesn't really NEED anything but 3.73s. 4.10s on 38+ wake it back up, but it is fine without.

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