New Dana Gears

13_gecko_rubi

New member
Thanks! That's a nice calculator.

With our nice 8-speed tranny the difference between 5.38 and 4.10 gears (all else being the same) boils down to what happens at very low and very high speeds; at moderate speeds the Jeep with 4.10s is just one gear lower than the one with 5.38s and who cares? But at high speeds the engine spinning the 4.10s is running slower, making less noise and probably getting slightly better fuel efficiency. At very low speeds the one with 5.38s is spinning faster making a bit more power. The one with 5.38s will probably win a drag race, but not by much. It will also crawl slower at a given RPM but again, not by much: 5.38s at 1500 in 1st gear low range on 37" tires will be going 1.6 mph, the one with 4.10s will be doing 2.1 mph. Does that matter?

I prefer lower RPMs at high speed. If I cared about acceleration I'd buy a Tesla :yup:
But what you dont realize is that your 3.6 cannot hold those low RPM at high speed with larger tires. My JK on 37s with 4.88s could never hold 5th at the slightest inkling of a grade it would drop to 4th. At even a mild grade it would drop to 3rd. If I were going to run 37s in my JL I'd do 5.38s in it. 5.13s would be as low as I would go. You need the 3.6 higher in rpm range where it makes torque to spin those heavy meats. Many folks try to run lower gears being their DDs thinking the will get better FE. Well the moment it drops from top gear down its worse than had u run deeper gears and held top gear.

Now the JL wide range trans ratios makes gear swaps not really necessary. You can run 4.10s all day with 37s and it is quite liveable. You just need to understand you now have a 6 or 7 speed, not an 8 speed. It will be hard for most to justify gear swaps. But if u do it do it right and hear for 8th.

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BillArnett

New member
... Many folks try to run lower gears being their DDs thinking the will get better FE. Well the moment it drops from top gear down its worse than had u run deeper gears and held top gear. ...

In my limited experience so far, I seem to stay in 8th when it's flat or downhill, ie about 1/2 the time. And even when I drop down to 7th I'm spinning at 2378 RPM at 75 mph which is comfortable enough whereas with 5.38s it would be 2489 RPM at 75 in 8th (according to the above calculator). The difference is insignificant but it should be very slightly more efficient with the 4.10s; even though I'm in 7th gear I still have a higher final drive ratio.

So maybe I have a 7.5-speed :)

The real reason I'm not going with lower gears is just that I don't want to spend a lot of money for what seems to me to be at best a very slight benefit. I'm going on a long trip next month. Maybe my opinion will change.
 

13_gecko_rubi

New member
In my limited experience so far, I seem to stay in 8th when it's flat or downhill, ie about 1/2 the time. And even when I drop down to 7th I'm spinning at 2378 RPM at 75 mph which is comfortable enough whereas with 5.38s it would be 2489 RPM at 75 in 8th (according to the above calculator). The difference is insignificant but it should be very slightly more efficient with the 4.10s; even though I'm in 7th gear I still have a higher final drive ratio.

So maybe I have a 7.5-speed :)

The real reason I'm not going with lower gears is just that I don't want to spend a lot of money for what seems to me to be at best a very slight benefit. I'm going on a long trip next month. Maybe my opinion will change.
Nah, u nailed it. Wont be worth it to most people. Gear companies are gonna hate the JL Rubicon owners. I bet most only swap gears if they swap axles.

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Amric

New member
I would think 4.88 would be the ideal gear for a JL on 37s. 4.56 gear would put 8th gear closest to what the RPM was at with the 33” tire, so going one gear up from there would give a little extra to make up for the extra rotating weight.

5.13 seems like the upper edge of RPMs I would want to be at for any long highway trips.
 

13_gecko_rubi

New member
I would think 4.88 would be the ideal gear for a JL on 37s. 4.56 gear would put 8th gear closest to what the RPM was at with the 33” tire, so going one gear up from there would give a little extra to make up for the extra rotating weight.

5.13 seems like the upper edge of RPMs I would want to be at for any long highway trips.
You cannot run even close to the same engine rpm on 37s as you did on 33s. They are too much heavier, the engine cannot hold that rpm.

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BillArnett

New member
You cannot run even close to the same engine rpm on 37s as you did on 33s. They are too much heavier, the engine cannot hold that rpm.

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Basic physics: When moving at at constant road speed (= constant wheel RPM) the *weight* of the tires is irrelevant, only their circumference. The rotating mass matters only for *acceleration*.

In the real world there are a lot more factors in play when thinking about gears. I'm sure the FCA engineers had good computer models with which to try a zillion combinations of tires, wheels, gears, and transmission algorithms. And then they did real world testing with the most promising combinations and picked the one that was optimal for whatever they were trying to optimize (probably fuel efficiency on the EPA tests). And then because we wanted to optimize for something else (eg rock crawling) we put on bigger tires and threw their careful optimizations out the window. :yup:
 

13_gecko_rubi

New member
Basic physics: When moving at at constant road speed (= constant wheel RPM) the *weight* of the tires is irrelevant, only their circumference. The rotating mass matters only for *acceleration*.

In the real world there are a lot more factors in play when thinking about gears. I'm sure the FCA engineers had good computer models with which to try a zillion combinations of tires, wheels, gears, and transmission algorithms. And then they did real world testing with the most promising combinations and picked the one that was optimal for whatever they were trying to optimize (probably fuel efficiency on the EPA tests). And then because we wanted to optimize for something else (eg rock crawling) we put on bigger tires and threw their careful optimizations out the window. :yup:
When you find that road where basic physics applies that is dead flat you are never accelerating for more than maybe a few feet let me know :) You are always accelerating and loading the engine or decelerating just based on friction alone. The rolling resistance of 37x12.5s or 13.5s alone is a huge increase vs the stock tires. And you are correct there is a ton of simulation and testing that goes into gear ratios, tire rolling resistance, transmission and torque converter calibration, aero, etc etc. On Sport and Sahara highly biased toward fuel economy. Rubicon not so much.

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BillArnett

New member
When you find that road where basic physics applies that is dead flat you are never accelerating for more than maybe a few feet let me know :) You are always accelerating and loading the engine or decelerating just based on friction alone. The rolling resistance of 37x12.5s or 13.5s alone is a huge increase vs the stock tires. And you are correct there is a ton of simulation and testing that goes into gear ratios, tire rolling resistance, transmission and torque converter calibration, aero, etc etc. On Sport and Sahara highly biased toward fuel economy. Rubicon not so much.

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Basic physics always applies everywhere. That's why it's called basic physics :) Unfortunately, a lot of not-so-basic stuff applies on top of that and can often be more important. (EG when cruising down the freeway most of the engine torque is going to over come aerodynamic drag.) I was only objecting to the idea that more massive tires require more force when moving at a constant speed. That's not true *everything else being equal*. If the more massive tires also have higher rolling friction then that's another of those not-so-basic things :)
 

BillArnett

New member
As I said before, do you live where the earth is totally flat?


Sent from my iPhone using JL Wrangler Jeep Forum mobile app

Far from it, actually. I'm in a very hilly area. I think that's part of the reason I'm getting somewhat worse mileage than others report here.

But basic physics still holds: the *weight* of my wheels does not matter if I go up and down those hills at a constant speed. (The JL's cruise control isn't good enough to actually hold a constant speed on my hills but that's a minor factor.)
 

13_gecko_rubi

New member
Basic physics always applies everywhere. That's why it's called basic physics :) Unfortunately, a lot of not-so-basic stuff applies on top of that and can often be more important. (EG when cruising down the freeway most of the engine torque is going to over come aerodynamic drag.) I was only objecting to the idea that more massive tires require more force when moving at a constant speed. That's not true *everything else being equal*. If the more massive tires also have higher rolling friction then that's another of those not-so-basic things :)
I'm an engineer too so yes I get it and theoretical physics applies almost no where. There are always 100 other variables.

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13_gecko_rubi

New member
Far from it, actually. I'm in a very hilly area. I think that's part of the reason I'm getting somewhat worse mileage than others report here.

But basic physics still holds: the *weight* of my wheels does not matter if I go up and down those hills at a constant speed. (The JL's cruise control isn't good enough to actually hold a constant speed on my hills but that's a minor factor.)
So you truly think that by maintaining a constant speed uphill you arent accelerating to counteract gravity decelerating you in opposite direction and your tire weight has no effect? Welcome to the real world now. You sound like a teacher...

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BillArnett

New member
Getting back to reality on a slightly different track...

If you swap you 4.10 gears for 5.38s you're multiplying the torque on the axles by 538/410 or about 30%. That is, at any given engine output/transmission gear selection/ transfer case setting 30% more torque will be applied to the axles. And therefore at full throttle in 1st gear in 4Lo the axles will get 30% more torque than they were designed for. Especially the U-joints. Are they strong enough for that? Especially if there's any kind of shock load applied in addition? Axles and U-joints are common failure points on the JK. Is the JL any better? One of the (minor) reasons I'm sticking with 4.10s (for now anyway) is that I'm not confident about those answers. I try to avoid full throttle and bouncing but if my foot slips or I hit an unexpected bump at the wrong time...

OTOH, if I find that I'm using full throttle very often when wheeling then I'll think about lower gears. And probably stronger axles, too. I hope not. That's way expensive.
 

13_gecko_rubi

New member
Getting back to reality on a slightly different track...

If you swap you 4.10 gears for 5.38s you're multiplying the torque on the axles by 538/410 or about 30%. That is, at any given engine output/transmission gear selection/ transfer case setting 30% more torque will be applied to the axles. And therefore at full throttle in 1st gear in 4Lo the axles will get 30% more torque than they were designed for. Especially the U-joints. Are they strong enough for that? Especially if there's any kind of shock load applied in addition? Axles and U-joints are common failure points on the JK. Is the JL any better? One of the (minor) reasons I'm sticking with 4.10s (for now anyway) is that I'm not confident about those answers. I try to avoid full throttle and bouncing but if my foot slips or I hit an unexpected bump at the wrong time...

OTOH, if I find that I'm using full throttle very often when wheeling then I'll think about lower gears. And probably stronger axles, too. I hope not. That's way expensive.
That is true. But the difference is that u dont have to apply as much engine load because more torque is going to the wheels like u said. If you go wot in either hopping probably gonna break either way.

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BillArnett

New member
So you truly think that by maintaining a constant speed uphill you arent accelerating to counteract gravity decelerating you in opposite direction and your tire weight has no effect? Welcome to the real world now. You sound like a teacher...

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I thought we were talking about "spinning" the tires, ie the angular momentum. Of course, the weight of the tire matters insofar as it's part of the overall weight of the vehicle. But that's a very small factor (my wheels+tires are about 120 lbs, stock was about 75. The whole Jeep with me and my stuff is >5000. So the additional weight of the tires is 45/5000 or less than 1%.)
 

BillArnett

New member
That is true. But the difference is that u dont have to apply as much engine load because more torque is going to the wheels like u said. If you go wot in either hopping probably gonna break either way.

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Right about engine load. But I'm coming from the old 3.8 in my JK; the new 3.6 seems like it has plenty of power to spare when off-road. (On the highway it's pretty wimpy but I knew that going in :)

I can't quite parse "If you go wot in either hopping".
 

13_gecko_rubi

New member
Right about engine load. But I'm coming from the old 3.8 in my JK; the new 3.6 seems like it has plenty of power to spare when off-road. (On the highway it's pretty wimpy but I knew that going in :)

I can't quite parse "If you go wot in either hopping".
Wot... wide open throttle

The 3.6 does great off road in 4lo even on 40s. Hwy, not so much. At least in my opinion lol.

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