Help me settle the regear ratio question

[Some Random Guy]

Stock Rubicon here. I feel the 4.10's are marginal. 6th only becomes useful 75+mph IMO. As smooth, quiet, and efficient the Pentastar is, running it at 2500-2800 on the highway really has no effect on comfort or mpg in my experience. I should note I have the SOT top which is not quiet on the highway.

Frankly the gear ratios in this trans are to widely spaced for the torque of the engine. I had a '79 Cherokee that I swapped a "wide ratio" NV3500 into. Those ratios were about perfect. Never seemed to lug the engine unless I did something wrong. The D478 has a wider ratio spread, which results in the not torqy engine lugging in a lot of scenarios.

@Some Random Guy , would you attribute your highway performance to gears or tire size?

Pete

I can use 6th at 55mph now. It is perfect for me, my commute’s max speed limit is 50 mph. If I moved to a more highway commute, gears would need to be changed to bring RPM’s down on the interstate.

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[R00STER]

That'd make for some steep driveshaft angles. It'd have to go between the trans and T-case to be useful in 4wd for increased low range (like a marlin crawler), and if he's already barely holding 6th, more overdrive wouldn't help.

Good points, but i think this would only work for the rear axles, and since it replaces the tail cone it is not supposed to change overall driveshaft length. So you couldn't turn a road jeep into a trail jeep, but you may be able to turn a trail jeep into a more fuel efficient road jeep in 2WD

Since these are electrically activated, you could just run it to an aux switch.

I point a feeler out to the vendor to see if they support this type of application, but it seems to be universal amongst chevy/ ford/ dodge.

 

[grimmjeeper]

Good points, but i think this would only work for the rear axles, and since it replaces the tail cone it is not supposed to change overall driveshaft length. So you couldn't turn a road jeep into a trail jeep, but you may be able to turn a trail jeep into a more fuel efficient road jeep in 2WD

Since these are electrically activated, you could just run it to an aux switch.

I point a feeler out to the vendor to see if they support this type of application, but it seems to be universal amongst chevy/ ford/ dodge.

Jeeps had those types of overdrives back in the day as well. And I haven't seen many modern GM/Ford/Dodge trucks with any add-on overdrives like we used to.

Modern transmissions, both automatic and manual, have much wider gear spread (difference between low gear and top gear). And that means they can do everything you couldn't do with the old transmissions (especially compared to non-overdrive transmissions). An old CJ3 with a T98/Spicer18 running 4.27 axle gears needed that auxiliary Warn overdrive unit to run over about 45 MPH on the highway. A modern JL already has enough ratios available to idle in traffic smoothly and run with a reasonable RPM at 75 on the freeway. All you need is to tweak the axle gear ratio when you change tire size and you retain all of that capability without needing an extra overdrive.

 

[R00STER]

Jeeps had those types of overdrives back in the day as well. And I haven't seen many modern GM/Ford/Dodge trucks with any add-on overdrives like we used to.

Modern transmissions, both automatic and manual, have much wider gear spread (difference between low gear and top gear). And that means they can do everything you couldn't do with the old transmissions (especially compared to non-overdrive transmissions). An old CJ3 with a T98/Spicer18 running 4.27 axle gears needed that auxiliary Warn overdrive unit to run over about 45 MPH on the highway. A modern JL already has enough ratios available to idle in traffic smoothly and run with a reasonable RPM at 75 on the freeway. All you need is to tweak the axle gear ratio when you change tire size and you retain all of that capability without needing an extra overdrive.

True, I had looked at this as an option for my old 70 challenger 4-speed with 4.10s. It got to 90mph quick, but it screamed once it got there.

I did talk to the guys at Gearvendors and they don't (and wont) have a solution for Jeeps. I would delete my posts, but it may stop someone else from going down this path. Sorry all.

 

[omnitonic]

Not to hijack, but with all the discussion of hitting the perfect ratios by messing with the rearend, has anyone consider a bolt on ubder/ overdrive unit like Gear Vendors? Not sure if they make one one jeeps, but it could potentially solve the age old question of fuel economy and off-road capability.

I did think about that, and there is nothing available for the JL. If Gear Vendors did make a unit for our application, their approach to 4WD is to put it behind the transfer case and lock out 4WD when it's engaged. Therefore, you'd have to gear the axles low and do an overdrive. This would mean the engine is pushing an overdrive at highway speeds. I have an Advance Adapters overdrive on an old F350. The gears are LOUD in overdrive, and the parasitic losses are so great the whole thing just about wasn't even worth doing.

More interesting is the Atlas transfer case for the JL, which could become a thing, but is not yet a thing. I'm keeping an eye out, but I won't be able to afford that either, so it is what it is. Apparently some of the old ones had three sticks with an extra deep reduction section, and they could get crazy stupid low ratios. My impression is that nothing that cool is on the drawing board for the JL.

 

[omnitonic]

5.13 on 39's... If I lived ANYWHERE I did more than 250 miles per month on the highway, this combo would suck... If you regularly drive around 80 mph, stick with the 4.10's, even on 35's.

I regularly drive 70-75, and I log 380 miles a week on the highway. I log up to 100 miles off pavement on the weekend. I stay out as long as I can and go out two days in a row whenever I have that much time free from chores. That's what makes it tricky to go for that perfect ratio. 4.88 would be way too low, I could live with 4.10 if I already had that ratio, but since I don't, I seem to be kind of aiming at the middle with 4.56. Some guys say 4.56, some guys say 4.10. Yeah, I know. If they made something right in the middle, that's where I would go.

Stock Rubicon here. I feel the 4.10's are marginal. 6th only becomes useful 75+mph IMO.
Frankly the gear ratios in this trans are to widely spaced for the torque of the engine.

Where the 8-speed auto has extra gears is down below the gear we call 4th. Having these extra steps would be very useful to me, as I deal with a lot of situations where I'm going up mountain roads revving high in a low gear, because as soon as I shift up, the bottom falls out. Having more ratios down in that range would be really helpful.

That's kind of what I end up doing with this regear. I'm sliding the top overdrive off the chart, and sliding everything else up a notch. I'm changing a bunch of other stuff along the way, and I'm not sure how it's going to play out, but it will be interesting to see. I may end up turning the same RPMs in a totally different gear ratio after the regear, and not really gain anything, or I might find that I have more options in that lower range where I'm only getting 9 mpg currently, screaming up the mountain roads in a ridiculously low gear, because I can't take the switchbacks any faster.

Time to drag myself away and go to work. I appreciate everybody's input so far. This has been an interesting discussion, although I'm not sure I'm any closer to settling on a ratio.

 

[GATORB8]

More interesting is the Atlas transfer case for the JL, which could become a thing, but is not yet a thing. I'm keeping an eye out, but I won't be able to afford that either, so it is what it is. Apparently some of the old ones had three sticks with an extra deep reduction section, and they could get crazy stupid low ratios. My impression is that nothing that cool is on the drawing board for the JL.

Not applicable to Jeeps, but cool if you're into this kinda stuff, Marlin Crawler does some interesting work with Toyota's similar to the Atlas method. Since the Yota T-case is modular, they sell add on "modules" that basically add another low range each with a separate stick. IIRC they're stackable. They have kits that'll do like 500:1 low range.

They have videos where they'd put a truck in super low range on a rock garden, let the clutch out and jump out of it and walk beside it.

Edit: Found one on the Rubicon:

 

[Some Random Guy]

@omnitonic
What might help push you off the fence is that the ring/pinion get weaker the deeper you go. So if you don’t gear as deep, you can be confident they’ll be strong on your mountain commute, but you’ll still see an improvement over stock.

 

[grimmjeeper]

I did think about that, and there is nothing available for the JL. If Gear Vendors did make a unit for our application, their approach to 4WD is to put it behind the transfer case and lock out 4WD when it's engaged. Therefore, you'd have to gear the axles low and do an overdrive. This would mean the engine is pushing an overdrive at highway speeds. I have an Advance Adapters overdrive on an old F350. The gears are LOUD in overdrive, and the parasitic losses are so great the whole thing just about wasn't even worth doing.

More interesting is the Atlas transfer case for the JL, which could become a thing, but is not yet a thing. I'm keeping an eye out, but I won't be able to afford that either, so it is what it is. Apparently some of the old ones had three sticks with an extra deep reduction section, and they could get crazy stupid low ratios. My impression is that nothing that cool is on the drawing board for the JL.

I had one of those 4 speed Atlas transfer cases on my manual transmission JK. They put a planetary gear set between the transmission and the gear driven part of the transfer case. The gear driven guts are mostly the same as their standard transfer case.

I got the 4.3:1 low range gear set for the transfer case. Combined, it gave me an 11.7:1 compound low range. With everything factored in, I was near 238:1 in low/low/first gear. Roughly 20 seconds for one tire rotation at idle. I mostly spent my time off road in 4:3:1 low range. But when I wanted it, I had the ability to crawl super slow and it came in handy in a few places.

The biggest issue with a case like that is dealing with the transmission. Some automatic transmissions have an output shaft speed sensor in them somewhere. If they do, the computer can directly figure out how and when to shift the transmission. If not, they have to rely on the wheel speed sensors and do some calculations based on low range to figure out how fast the output shaft on the transmission is spinning. If you have multiple low ranges, your computer doesn't know what low range to use when doing that math so it can't figure out when to shift and to which gear. And there's nothing you can easily do to the computer to fix that. A manual transmission is a lot easier to deal with in that regard since there isn't a computer in charge.

But the 4 speed Atlas is just a very expensive case with fewer and fewer vehicles capable of taking advantage of it. I wouldn't be surprised if it goes away completely at some point.

 

[grimmjeeper]

Not applicable to Jeeps, but cool if you're into this kinda stuff, Marlin Crawler does some interesting work with Toyota's similar to the Atlas method. Since the Yota T-case is modular, they sell add on "modules" that basically add another low range each with a separate stick. IIRC they're stackable. They have kits that'll do like 500:1 low range.

They have videos where they'd put a truck in super low range on a rock garden, let the clutch out and jump out of it and walk beside it.

Edit: Found one on the Rubicon:

I did a little bit of that when I installed my 4 speed Atlas testing it right after the install. It was in my driveway but still...

 

[grimmjeeper]

Also, Off Road Design has their Magnum box that bolts to the front of an NP205 transfer case. It does 1:1, 1.96:1, 2.72:1 and 5.33:1.

https://www.offroaddesign.com/offroad-design-magnum-underdrive.html

I'm seriously looking at putting one in my Comanche.

 

[DanW]

I have the 3.6L 6MT and 3.45 gears. This table neatly summarizes my options:

Running at 70 mph​

	4th	5th	6th
3.45 (current)	2531	2055	1822
4.10	2755	2232	1984
4.56	3064	2482	2206

Sitting on my current wheels and tires, I'm getting an overall average of 20 mpg I currently use 6th and 5th on the way to work, and 5th and 4th (and occasionally 3rd) on the way home. The grades are much steeper on the way home.

It looks like 4.10 is a good compromise in terms of fuel economy. I get my cruising RPMs up a lot closer to the sweet spot at 2000 while still having some room to lug along if I want. If I already had 4.10 gears, I would probably just stay there.

If I go to 4.56, I end up turning a little high in any of the gears I would normally use at 70 mph. I currently spend a lot of time in 5th, so I guess it's really only about 150 RPM. On the other hand, when I currently do downshift to 4th, I can see those fuel economy numbers drop like a stone, and that is only 300 RPM higher than I would be running in the new 6th gear.

One of the things that's making this kind of hard to puzzle out is that there is basically no flat stretch of road anywhere around here so I can cruise at 70 on flat ground in each of the three gears. I notice I get worse gas mileage at wide open throttle than I do if I'm at, say, 50% throttle. Driving around in something close to the equivalent of my current 4th gear all the time might not be as bad as I think. I am currently only seeing the fuel economy plummet whenever I'm in a wide open throttle situation. At other times, I am normally in 5th or even 6th.

So that's why I'm writing all this up. I'm looking for folks to share experiences they've had. I have a feeling y'all are going to talk me into the 4.56 gears. I'm leaning that way, but I need more convincing.

I just went from 4.10 to 4.56 gears, both on 35's. To me, it was well worth it. It made 6th gear much more useable and made the Jeep feel lighter and quicker all around. I would have been fine staying with 4.10's, but 6th wasn't getting used much. If I had a head wind or slight incline, it would hold 6th, but gas mileage was better in 5th on those conditions.

If running 35's and spending the money, I'd go with 4.56 and wouldn't bother with 4.10's. The 4.56's really hit the sweet spot, for me.

I went with Dana/Spicer rings/pinions and overhaul kits. I've got about 500 miles on them with no issues or noises of any kind, so far.

It is at about 2400 or 2450 rpm at 75mph, which is just right. (Corrected! I had mistakenly typed 4400 or 4450. Thanks, @omnitonic!)

 

[CarbonSteel]

@omnitonic
What might help push you off the fence is that the ring/pinion get weaker the deeper you go. So if you don’t gear as deep, you can be confident they’ll be strong on your mountain commute, but you’ll still see an improvement over stock.

Even at 5.38 unless you are running 40's and SERIOUSLY abusing it, the ring and pinion are not the weakest link.

 

[Some Random Guy]

Even at 5.38 unless you are running 40's and SERIOUSLY abusing it, the ring and pinion are not the weakest link.

True, just trying to help OP get off the fence. The gear struggle is real, especially in the M/T.

 

[Kreepin1]

Even at 5.38 unless you are running 40's and SERIOUSLY abusing it, the ring and pinion are not the weakest link.

Enlighten me. What is weaker than a 5.38 gear set? Assume I'm running 37's.

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