Re Gearing to 4.10

[Pete M]

27 minutes ago, 89 MJ said:

 especially if you want to go on the highway at all.

 

RPMs are not a straight-up indication of MPGs.  the 2.5L/auto came with 4.56 gears stock.   

[derf]

 

There's a sweet spot for an engine in a given vehicle.  It has to produce enough torque to move the vehicle down the road.  Torque is affected by gearing, both in the transmission and the axle.  The size of the tire affects the torque and how it's applied to the ground.  Obviously, high RPMs will reduce mileage.  But dropping the RPMs too low can also reduce mileage.  With tall gears (lower numbers) the engine has to produce more torque to move the vehicle.  That means the engine is doing more work and that means it's consuming more fuel.  There's basically a sweet spot.  An ideal RPM range where the engine is providing enough torque through the transmission and gears to move the vehicle.  Above or below that, the MPGs drop off.

 

The manufacturers choose gearsets for various reasons.  One of the chief concerns is mileage on the EPA test loop.  This is why the XJ/MJ has 3.55s with the 4.0 auto instead of 3.73s or 4.10s.  It's why the XJ/MJ 4.0 manual came with 3.07 gears instead of 3.31 or 3.55s.  But the 2.5 manual and auto both came with much shorter (higher number) gears.  It's because that engine produces less power and needs the torque multiplication of shorter gears in order to meet performance expectations.  That's also why it doesn't get dramatically better mileage than a 4.0.  A little bit, maybe.

[89 MJ]

51 minutes ago, Pete M said:

 

RPMs are not a straight-up indication of MPGs.  the 2.5L/auto came with 4.56 gears stock.   

You make a fair point yet again. I was getting at the fact that my truck (although not running right) was pretty much maxed out at 70 mph.

[Pete M]

with a properly running engine it should be able to get past 70 with either 4.10s or 4.56 on 33s.    the key here is what it feels like when starting from a stop.  a bit more gearing will make the Jeep feel alive and powerful.  

[JMO413]

12 minutes ago, Pete M said:

with a properly running engine it should be able to get past 70 with either 4.10s or 4.56 on 33s.    the key here is what it feels like when starting from a stop.  a bit more gearing will make the Jeep feel alive and powerful.  

I know that going from 3.07 to 4.10 with 31's was a HUGE difference. Felt like I added 2 more cylinders.

[gogmorgo]

1 hour ago, JMO413 said:

I know that going from 3.07 to 4.10 with 31's was a HUGE difference. Felt like I added 2 more cylinders.

33% reduction means a 33% increase in torque to the wheels. 33% of six is two more cylinders. 

 

One more thing that hasn’t been mentionned, is that if you go too low with a manual, then first gear stops being useful unless you really need the torque for a hill start or a heavy load. Not that there’s anything wrong with turning a five-speed transmission into a granny gear four-speed without overdrive, but it’s something else to consider. 
I know when I went to to the 31s the loss of off-the-line torque was more than negligeable, but the other effect of the taller effective ratio was that I could get all the way across an intersection before I needed to shift out of first, and to me that’s an improvement. It’s also a thing that sports car manufacturers have been known to do. If the engine makes enough torque, you can make your 0-60 times better if 1st is tall enough you don’t need to shift before you hit 60.

[JMO413]

4.10 made my first great a little short. It isn't granny but it's a little short. It is great for a nice off-road cruise speed for sure. Fifth is also a useable great now. With the 3.07 the truck might keep speed going down hill with a tail wind. 

[scguy]

What are y'alls thoughts on 4.10s  vs 4.56s and 32s? AW4, I'm going to be doing axles soon, 4.56s would cost a good bit more since I'd have to regear the C8.25 also. 

 

 

[Pete M]

costs can be a huge factor in this.  if I had to pay for it all, I'd do 4.56 in a heartbeat since it's essentially the same price.  BUT if I could cut that bill in half... then 4.10s are looking miiighty good.   

[derf]

52 minutes ago, scguy said:

What are y'alls thoughts on 4.10s  vs 4.56s and 32s? AW4, I'm going to be doing axles soon, 4.56s would cost a good bit more since I'd have to regear the C8.25 also. 

 

 

I have 265/75R16 (32s) with 4.56s in my XJ with the 4.0/AW4.  It's just about perfect up here at altitude.  We have a lot of hills around here and I wheel it up in the mountains.  It's a good balance.

[scguy]

Not counting other costs, like, gear oil, SOA conversion stuff, brake lines, etc I can buy a Chrysler 8.25 with 4.10s for $150 and a TJ low pinion D30 with 4.10s for the same, $150. That's $300 to go from 3.55s to 4.10s.

Now, I could regear my D30, but ring, pinion, install kit, new carrier pushes it to ~$350-400, installing myself. Pick up a cheap 3.55 8.25 for $100, same deal, ring and pinion setup, install kit, $275. So, at the cheapest, $725-775 for 4.56s with a lot of labor.

Decisions, decisions.

[Smokeyyank]

Buy once cry once. I'd go 4.56s. When I had 4.10s I always wished it was deeper. 

[Jesse J]

while we are on the topic whats yalls opinion/experience with lunchbox lockers?

[derf]

They are a cheap way to get traction.  They vary in how much you notice them in your Jeep.

 

They do give you more traction and are a cheap way to improve off road driving capabilities.

 

If you drive on icy roads at all, they are not good.  They do more harm than good, especially in the front.  For a snow/ice driver, I'd prefer an open differential over any automatic locker.

 

I prefer the TrueTracs in my XJ (with the full time capable NP242) for a snow/ice capable driver.  I had E-Lockers in my JK and it was great in the snow as well.  Leaving the lockers unlocked until you need them is an option I am a big fan of.  But they're not cheap.  But buy once, cry once applies.

[Jesse J]

yeah that makes a lot of sense. I think once I have taken my truck wheeling many times to see if I a. need more traction and b. If I have enough cash or want to buy e lockers or lsds

[Muncher]

9 hours ago, Pete M said:

If you're worried about MPGs, don't daily a lifted brick on 33s.   

 

the 4.10s are... tolerable.   Nothing more exciting than that.  I only ran them because my budget didn't allow for a regear.  I still want a bit more oomph to get going from a start. 

a rounded brick*

[Muncher]

7 hours ago, gogmorgo said:

My plan for my mj has also been 4.10 and 33’s. To match the same ratio as the factory 3.07 and ~29” tires on 33’s you would only need 3.73 gears, which is roughly the same as running 31’s on 3.55 gears, as I’m doing in my manual ZJ. If you need deeper gears offroad, that’s what low-range is for. If I was towing regularly with it I can see wanting more gear, but it’s fine as a daily. It’ll run out of traction before it runs out of torque to climb hills, and a 3000rpm clutch dump launch and WOT acceleration still scares passengers who weren’t expecting it (it hooks hard and then goes) and I have no issues comfortably keeping the revs between 1000 and 2500 rpm under normal driving. It’ll still pull most hills on the highway in cruise in 5th gear without losing speed, and I live in the Rockies, so there’s lots of them and they’re not always gentle.

Keeping the revs down at highway speed also goes a long way for comfort on long drives.

 

But that all said, if you think you ever might go beyond 33’s, going with deeper gears will have you ready for that, and won’t be unmanageable in the interim, and you’ll only need to pay to regear once. It’ll also be helpful if you’re hauling around lots of extra weight in the form of armour or camping gear. 

i don't ever want to go above a 33. id get a wrangler if i was going to want bigger tires. 

[Muncher]

2 hours ago, scguy said:

Not counting other costs, like, gear oil, SOA conversion stuff, brake lines, etc I can buy a Chrysler 8.25 with 4.10s for $150 and a TJ low pinion D30 with 4.10s for the same, $150. That's $300 to go from 3.55s to 4.10s.

Now, I could regear my D30, but ring, pinion, install kit, new carrier pushes it to ~$350-400, installing myself. Pick up a cheap 3.55 8.25 for $100, same deal, ring and pinion setup, install kit, $275. So, at the cheapest, $725-775 for 4.56s with a lot of labor.

Decisions, decisions.

wouldnt a 8.25 be a downgrade from a Danna 44? 

[scguy]

1 minute ago, Muncher said:

wouldnt a 8.25 be a downgrade from a Danna 44? 

I've heard they are similar, at least the 29 spline 8.25. However, I'm replacing a very worn Dana 35 with bad bearings. 

[derf]

2 minutes ago, Muncher said:

wouldnt a 8.25 be a downgrade from a Danna 44? 

They're both in the same class.  Almost the same strength if you have the 29 spline 8.25.  upgrading a 27 spline is a matter of swapping shafts and the differential.

[Muncher]

1 minute ago, scguy said:

I've heard they are similar, at least the 29 spline 8.25. However, I'm replacing a very worn Dana 35 with bad bearings. 

ohj ok. also wouldn't i need to put 30 spline axles in anyways to re gear?

[Muncher]

so it seems like just getting different axles might be the more cost effective route. is the ford 8.8 a axle that will work?. and if so what should i use for a front axle that would have 4:10 

[Muncher]

Or could something like a Cherokee axle be a bolt in? that would have 373s 

[ghetdjc320]

10 minutes ago, Muncher said:

so it seems like just getting different axles might be the more cost effective route. is the ford 8.8 a axle that will work?. and if so what should i use for a front axle that would have 4:10 

 

The 8.25 is better for 33's than an 8.8 imho. 8.8 is just too narrow for MJ's and too fat of an axle for 33's. Add the thin axle tubes, and c-clips and its really not a great option compared to the 8,25. I run a super 35 that works great with tons of clearance.  For a front axle, most 4cyl D30 fronts will have 4.10's. Look for an XJ or MJ 4.10 D30 front (ideally without the vacuum disconnect-but your front driveshaft will now spin constantly). A TJ axle will also bolt in but is a low pinion axle and would be a downgrade. If you're going with the regear option, go 4.56. You won't regret it. And if you swap axles, that old 44 will have some resale value to help cover the costs.

[Muncher]

tj axles are hard to find but maybe ill be able to get a 4 cyl one