ECODIESEL DIFF RE-GEARING

[AC77]

i went to 35s on the stock gearing, and it wasn't bad, though 4.10s would have been numerically nearly perfect.
when i went to 37s, i hated it until i was able to get it regeared and went to 4.56s which were numerically a little deep for 37s but would be nearly numerically perfect for 39s. 4.56s also help compensate for the extra weight and wind resistance of the larger tires and lift.

i feel 'driveability' is much improved over the stock gearing, and now i have all my fluids (cooling, lubrication and transmission) moving at at least the rates they were designed to be at for any given load.

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[grimmjeeper]

I don't disagree with the consensus here. I’ve always been curious though….. stock diesel sports had a 245/75/17 tire And 3.73. If Jeep intended that to be the “right” gear ratio for economy etc then for 35’s we would want 4.15 (so 4.10), and 38’s would want 4.50 (so 4.56). If we think Jeep engineered the diesel gearing for the Rubicon’s though, then 35’s would want a 3.99, and 37’s would want 4.33. Just depends on which tire spec they designed the diesel to.

Personally I think the diesel is good for 33’s easily, 35’s for flat landers who aren’t rock crawling or towing, as many here have proven.

I don't think Jeep intended 3.73 to be the "right" gear. But it was closest they could get.

They couldn't use the M186 because it isn't rated for that much torque. The M210 was needed. The tallest (lowest number) ratio for that axle is 3.73. I suspect the diesel would have come with 3.45 if the M210 had that ratio available.

 

[zouch]

numerically perfect: final drive ratio equal to original after compensating for change in tire diameter.

yes.
there may be a compromise, but they’ve got a target they’re aiming for. i’m for staying at least as close to that as they did.

how are your new gears working out?

What do you mean by numerically perfect? Are you under the impression that what's chosen is more than a crapload of compromises that includes what suppliers happen to be producing at a reasonable cost?

 

[zouch]

wheel diameter isn’t really relevant in terms of gearing ratios; it’s Tire Outer Diameter (O.D.) we need to be concerned with.

Thanks for this breakdown, my Sahara altitude came stock with 18's

 

[grimmjeeper]

numerically perfect: final drive ratio equal to original after compensating for change in tire diameter.

yes.
there may be a compromise, but they’ve got a target they’re aiming for. i’m for staying at least as close to that as they did.

how are your new gears working out?

That's based on a big assumption. The assumption that the starting point was a good ratio to begin with.

 

[Ratbert]

how are your new gears working out?

They have the same initial (minor) hum when letting off the accelerator as the last 4.63s. No clue if that's a sign of things to come or not.

This is the first iteration where anyone has said anything about breaking them in, so my wife is paranoid about taking them off road until we hit that 500 mile mark. That is, we've passed on Jeeping opportunities for the past two months. I drive it very little on road, so it's taken two months to officially break them in. Hopefully one of the Jeep clubs / groups will be wanting to do something this weekend!

 

[Ratbert]

That's based on a big assumption. The assumption that the starting point was a good ratio to begin with.

Now you can spec various gear ratios when you're ordering one. The concept of "perfect" clearly doesn't apply when it varies dramatically based on a check box on an order sheet. It may be perceived to be perfect for that person's use case, but perfect overall? Nope.

 

[zouch]

i'm OK, with assuming that numerically equal isn't going to be any *worse* than original.

That's based on a big assumption. The assumption that the starting point was a good ratio to begin with.

 

[grimmjeeper]

i'm OK, with assuming that numerically equal isn't going to be any *worse* than original.

Except that when you go up in tire size, you want to go a little further than stock to retain performance.

Compensating for diameter change gets you close. But there's the rolling resistance of a bigger contact patch, added wind resistance, and added unsprung weight. Granted, all of that combined is < 10% of the equation but it's not zero.

 

[Ruby Mike]

I have the 2020 JLYRD Wrangler with 37 inch tires. I am using the stock 3.74 gears and am having zero issues. I get over 22 mpg and the jeep only drops into 6th gear on steep freeway grades. I honestly don't see the need to regear.

 

[zouch]

right; we've already been over that.
and one of the reasons why i'm on 4.56s for 37s. (even 38s would be a little 'under' numerically equivalent, and probably perfect for *my* tastes.)

Except that when you go up in tire size, you want to go a little further than stock to retain performance.

Compensating for diameter change gets you close. But there's the rolling resistance of a bigger contact patch, added wind resistance, and added unsprung weight. Granted, all of that combined is < 10% of the equation but it's not zero.

 

[Ratbert]

right; we've already been over that.
and one of the reasons why i'm on 4.56s for 37s. (even 38s would be a little 'under' numerically equivalent, and probably perfect for *my* tastes.)

Many of us regear not to perfectly match the set of compromises put upon Jeep corporate, but instead to actually make it better based on our needs / desires.

By sticking with "perfect" numbers when going to larger tires you're reducing your rig's performance. Most of us, when putting thousands of dollars into tires and gears would prefer to not reduce performance and hopefully instead maybe increase it a bit.

It's all a compromise. I'm pretty sure there's no perfect compromise.

 

[grimmjeeper]

Many of us regear not to perfectly match the set of compromises put upon Jeep corporate, but instead to actually make it better based on our needs / desires.

By sticking with "perfect" numbers when going to larger tires you're reducing your rig's performance. Most of us, when putting thousands of dollars into tires and gears would prefer to not reduce performance and hopefully instead maybe increase it a bit.

It's all a compromise. I'm pretty sure there's no perfect compromise.

And that goes back to my assertion that with stock tires, the 3.73s were over geared. But they had no option to go taller on the gears to start with. Hence the compromise from the start and "perfect" isn't an apt description.

But if he likes his gear and tire combo, more power to him.

 

[Ratbert]

But if he likes his gear and tire combo, more power to him.

Absolutely. Everyone has things that they like or dislike. I'm just surprised that some people seem to have the perception that a given ratio for a given set of circumstances was somehow deemed to be "perfect" and somehow that perfection perfectly transitions to a different set of circumstances.

 

[zouch]

don’t get so hung up on “perfect”; it isn’t going to be the same for everyone.

for example; we don’t all have/want the same Xfr Case Low Range ratios.; one reason i did not want a Rubicon with the diesel is i preferred the 2.72:1 ratio instead of the 4:1 of a Rubicon.

a numerically perfect correction for my (current) tire size would have been closer to something around 4.32, but wasn’t an option from any reputable supplier.

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