To 4.10 or not

Clubbs

Well-Known Member
Joined
Jun 15, 2008
Location
Blounts Creek
Truck runs well with its new tune and a few less parts to breath through. Has this new phenomenon where is seems to get better mileage if I hold it down a gear now. Wasn't the case before and the trans tune is apparently set this way, but it can't make it's mind up between 5 and 6 at my usual 63mph cruise speed on my daily commute.

Rear end started to get some slop last year and now clacks and clanks with throttle off / on. Other than the noise I don't think it's a problem, all my other trucks have done the same and the poor beaten 14bolt in the CJ is horribly loose and still rolling.

Regardless, seems my factory 3.42's are holding the truck back some. Anyone running 4.10's on 35's with a tuned Cummins? Anything negative to be aware of? I imagine 1st gear will be all must useless unless towing, but based on current rpm to speed to mileage I think it will be happier with more revs and the trans tune will probably be better with this ratio too. Any thoughts?
 
4.10’s and 285/75’s on a stock Cummins. 70mph at around 2000rpm give or take. It rarely downshifts while unloaded but it’s also the Aisin.

You’re correct. 1st is about useless and it will shift up into 2nd before you’re through an intersection. But it doesn’t care what gets put behind it when loaded.
 
I have a ‘04 6 speed on 35s with a Smarty that’s usually on level 3. Actually I don’t know exactly what gears are in it. It pulls hard so I’m pretty sure it’s 4.10. If it is 4.10 I wouldn’t want 3.73s in it. I never touch 1st unless I’m pulling a decently heavy load. You’re not going more that 20ft in 1st.
 
Do it. I went to 4.10s in my 15, wouldn't have it any other way.

Duane
 
My 2019 with 68RFE has 3.73's with stock tires which are about 33" tall, so fairly close comparison to 4.10's and 35's. I wish mine had 3.42s for better fuel mileage. Not that the 3.73's are bad, just spins too many RPM on the highway and there's no shortage of power. But most of my highway miles are at 80-85mph, not 63mph, so that would have a significant effect on what I wanted too.

Is yours an Aisin or 68RFE? If its a 68RFE, theres about a 30% rpm drop from 5th to 6th gear, and going from 3.42 to 4.10 would give you a ~20% rpm difference, so running 5th all the time would give you some idea, but not a great comparison. If its an Aisin, there's about a 22% drop, so just running 5th would be a good simulation for 4.10s.
 
jeepinmatt said:
My 2019 with 68RFE has 3.73's with stock tires which are about 33" tall, so fairly close comparison to 4.10's and 35's. I wish mine had 3.42s for better fuel mileage. Not that the 3.73's are bad, just spins too many RPM on the highway and there's no shortage of power. But most of my highway miles are at 80-85mph, not 63mph, so that would have a significant effect on what I wanted too.
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Our trucks aren't exactly apples to apples comparison, but my truck gets better mileage at 80mph/1900ish RPM than it does at 70mph/1650ish RPM. Not significantly better, and I didn't do super accurate testing, but just something I've noticed over the years.

Duane
 
Mine is a 68rfe. With the current CCS tune it likes more turns. With the previous CPS tune it was marginally happier lugging than today. So it's all a factor of injection timing I suppose, and I wouldn't be considering a change if the rear end wasn't make the irritating sloppy clackety clack noise.

It spends the majority of its life between 60 and 65 mph. Even at 75mph the lie o meter says 19 in either 5th or 6th. At 63 I'm turning 1950rpm and seeing 21 for fuel (probably not accurate but apples to apples if I force it to 6th it's drops 2-3 mpg).

When towing (once a week typically) I'm either dragging the boat (26k gross) or my gooseneck loaded similarly.

Today 5th seems a little too low at 65 unloaded and 6th is much too tall. When towing at 65 the truck likes 4th but again it's too low and 5th is too tall unless I'm at or over 70 and then it's working pretty hard to sustain at those weights. Pulling hills loaded up to West VA it's either lugging in 5th and loosing speed or screaming at 2800 and holding 70mph ish, but can't shift to 5th.

I figure if 4.10's are too low I'll switch to 37" tires when I wear these 35's down. Ultimately everything should be on 40's it's just a process - although 40" mickeys on the boat trailer would create new challenges at the ramp...
 
It seems like 2000ish cruising rpm is the sweet spot. I always wanted a 2 speed rear... 4.10 for towing, 3.54 or maybe 3.08 empty. Double OD would be a good alternative... Just something to get empty rpms down a bit when running 70 or over.

The difference in power between 35s and 33s with 3.73 was surprising.
 
Clubbs said:
Today 5th seems a little too low at 65 unloaded and 6th is much too tall. When towing at 65 the truck likes 4th but again it's too low and 5th is too tall unless I'm at or over 70 and then it's working pretty hard to sustain at those weights. Pulling hills loaded up to West VA it's either lugging in 5th and loosing speed or screaming at 2800 and holding 70mph ish, but can't shift to 5th.
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Sounds like you need more gearing ;)
 
In the last couple of weeks I test drove some 2024 Ram 3500s, one with 4.10 and one with 3.73. I really liked the 4.10, and if I end up buying a new truck I will probably go that route depending on availability. The rpm difference at 70 mph was about 200 or 250 rpm, but it was still several hundred rpm less than my current straight drive Ram with 3.73 gears.
 
thecj3man said:
In the last couple of weeks I test drove some 2024 Ram 3500s, one with 4.10 and one with 3.73. I really liked the 4.10, and if I end up buying a new truck I will probably go that route depending on availability. The rpm difference at 70 mph was about 200 or 250 rpm, but it was still several hundred rpm less than my current straight drive Ram with 3.73 gears.
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FYI the 2025s come with an 8 speed tranny.
 
jeepinmatt said:
FYI the 2025s come with an 8 speed tranny.
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That and being very apprehensive of payments and new truck technology in general has me waiting on the sidelines. I bought my 06 2500 new and its been a great truck. The horror stories of new truck emissions scares me, but I know that it cant be all bad and I could eventually tune it. If I didn't want a DRW I would probably keep my 06 forever, but our new camper is creeping up to the axle weight limits and is already over slightly on payload. I have been able to move the load around the camper and stay close. But, I foresee a larger camper in the future too.

The newer trucks power and NVH are amazing compared to my 06 model.
 
Am I going to need a carrier change going from 3.42 to 4.10?

Any thoughts on installing an arb in the rear or can I keep the factory limited slip?
 
I think the carriers are fine. @Jody Treadway should be able to verify.

ARBs are cool, but it's either open or spool. Plus, they're like 1200+ doll hairs...as with almost every traction device for an AAM 11.5 or 11.8. I wish they'd get an E-Locker for them though, that'd be sweet. Limited slip when off, locked when on.

That said, I want to throw 4.10s in my 02 as well. My mileage never changes from 65 to 90 mph and it would put me at 1900-ish RPM at 65 mph and the non VGT turbo would be much happier when towing.
 
Clubbs said:
Am I going to need a carrier change going from 3.42 to 4.10?

Any thoughts on installing an arb in the rear or can I keep the factory limited slip?
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No carrier break on them. I'm literally about to re-gear a 2017 Cummins 2500 to 4.10 and adding an ARB up front this afternoon. Keeping the OEM TrueTrac thingie out back. Dooder just wanted more available traction up front as needed.
 
Croatan_Kid said:
I think the carriers are fine. @Jody Treadway should be able to verify.

ARBs are cool, but it's either open or spool. Plus, they're like 1200+ doll hairs...as with almost every traction device for an AAM 11.5 or 11.8. I wish they'd get an E-Locker for them though, that'd be sweet. Limited slip when off, locked when on.

That said, I want to throw 4.10s in my 02 as well. My mileage never changes from 65 to 90 mph and it would put me at 1900-ish RPM at 65 mph and the non VGT turbo would be much happier when towing.
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Swap in a Sterling or D80 from a Ford maybe? Pretty sure the factory E-locker ones are also limited slip. Super abundant, probably available from the factory with a 4.10 ratio in a gasser or something. Redrill the hubs, probably find a rotor that would be close enough from an older Ford/Chevy/Dodge that's 8x6.5, driveshaft yoke would be simple. Leaf spring perches are simple, shock mounts should be similarly easy. Biggest challenge would be ABS if you plan to keep that, but even that may be simple since most are just a 2 wire sensor.
 
CLRracer said:
I haven't been stuck yet, and I've been in some pretty messy job sites and landfills. Factory limited slip in the rear and open front. If I needed more traction I'd put the ARB up front IMO.

Duane
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Lemme know when you're ready😘
PXL_20250206_161450727.jpg
 
CLRracer said:
Our trucks aren't exactly apples to apples comparison, but my truck gets better mileage at 80mph/1900ish RPM than it does at 70mph/1650ish RPM. Not significantly better, and I didn't do super accurate testing, but just something I've noticed over the years.

Duane
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years of industrial applications have targeted 1800 rpms as the sweet spot (this is the rpm specified by power generation and pump applications alike ....so manufacturers spend a lot of time focusing on this range). It has been my experience any engine that has any industrial app will thrive 1800-2000 rpm.

ignore this for all power strokes, because they have no industrial application.
 
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