Who is running 37"-40" tires on the diesel

Sank

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Regearing returns engine rpms at a given speed to optimum (factory) levels, NTM reducing strain on the drivetrain.
The question isn't whether the diesel can perform without a regear; it's: can it perform optimally without a regear, and that answer is no.
I actually prefer the lower torque at the axles that the taller factory gears provide. When you gear down to 4.56 (for example) you have more torque at the wheels and I'm quite happy to avoid twisting axle shafts running 37s on factory d44s.

As for mpg, I get 22 if I'm nice, 23 if I baby it (manual shifting and lots of neutral coasting) or 21 mpg if I just don't give a care. My commute is almost all under 55 with few stops, at mile high altitude. Pretty calm. (Driving cross country on factory 33s and no lift i got 25-27 mpg).

What's optimal for you is not optimal for me.

Moab last month on 37s and 3.73 was totally fine. Will not regear.

 

Sbro2021

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I can attest to the detriment I have seen since throwing on 40's and stock gears.
Main issues:
Mileage loss- I drive ~800 miles per week and can tell all I went from maintaining 26mpg average consistently in stock form, 20mpg is hard to keep on the cluster.


With that loss is only one conclusion which is obvious, turning all that extra rolling mass takes more energy.... Which the consequences of that is also concerning. I regularly see coolant temps above 220, tough to keep below 210 against the consistent 203 I had seen prior to the new shoes. Bottom line the engine is throwing more fuel in response to the additional load and increasing EGT's which is not good in a diesel.

In addtion to the heat, the engine is not running at it's ideal rpm and essentially lugging all the time. That 300 RPM reduction is impactful on a diesel that is designed to run ~2000 rpm, also increasing EGT's. The cooling system cannot handle that much heat input.


All of that now consistent heat will only lead to reduced oil life, reduced life of all that plastic under the hood not unlike the fuel line for example in the 3.6 that seems to try to start engine fires eventually from all fo that heat cycling.( Almost started my JK on fire when it failed)

My plan is to regear to 4.56 or 4.88 to reduce that load and return to a "stock" ish ratio relative to a 40" tire. I also pull an Opus lite and want to keep that temp under control, along with possibly adding an additional remote on demand oil cooler to further help keeping those engine oil temps closer the coolant temp under load.

Would love to hear if anyone is having a similar experience and possible solutions to any of it.

-Cheers!
 

Pescatoral Pursuit

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When you gear down to 4.56 (for example) you have more torque at the wheels and I'm quite happy to avoid twisting axle shafts running 37s on factory d44s.
That's the opposite of how torque application works.

Think of two sledgehammers that weigh 5lbs each resting on a table. One has a 2' handle and the other a 4' handle. Laying your arm on the table you grab one at a time and lift them vertically by the base of the handle. The shorter handle is easier because of mechanical advantage. The strain on your hand/muscles/ tendons/ ligaments is greater with the longer handled sledge. It's the same with a higher gear ratio. It increases mechanical advantage thus reducing strain. Think of the piston on compression stroke in relation to the rollout of the tire. The length of compression stroke is fixed, let's say 4". Let's say the rollout on a 33" tire is 4"and on a 37" tire is 5". That same piston stroke has to work harder (along with the rest of the driveline) to move the tire 5" than 4".

What's optimal for you is not optimal for me.
It has nothing to do with you or me. The factory engineered the engine/ drivetrain/ tire size to work optimally together within certain parameters.
 

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Ram are available with 3.21, 3.55, and 3.92 ratios. A Rubi on 37's is very similar to a Ram spec'ed for fuel efficiency rolling with 3.21's? Some of the Rams come with a 34"+ tire,yes?

My 4.10's and soon to be 38's would be like a Ram on 3.55's kind of splitting the difference? Not like we have the towing/GCVWR capability of a Ram anyway?
 


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37’s on 3.73 with a Banks PedalMonster and Derringer … it’s blast! That’s not to say I am opposed to regearing (threw 5.13s on my LJ with 35s over a decade ago) but I don’t think I need it yet. Will be towing an off-road trailer so we will see how I feel after that but even then I’ll have to be realistic about the amount of time this Jeep sees as hard of stuff as my LJ to warrant it.

Hit 80 on the freeway and it keeps going fine but I’ve found 65 is right about the mpg sweet spot … but you should never buy a Jeep if your #1 goal is to see good mpg.
 

Sank

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That's the opposite of how torque application works.

It has nothing to do with you or me...it's engineered to work optimally together within certain parameters
I may be a nobody on this forum, but I think you merely misunderstand me. Respectfully, I don't really want to debate it in this thread beyond pointing out two simple things:

1. Torque multiplication. With a change in gearing, some amount of stress moves from one side of the gears to the other. I would rather add the stress to my driveshaft and transmission than my stock d44 axle shafts. I believe there is more margin in those designs than in the axle shafts.

2. You don't know my loads and speeds. Designing for "certain parameters" is nice but by definition they can never be universally applicable parameters. Every engineering decision is a delicate balance of competing requirements.

I appreciate good design and engineering as much as the next guy. Feel free to PM me to discuss physics and engineering further, outside of this thread.

In addition to the heat, the engine is not running at it's ideal rpm and essentially lugging all the time. That 300 RPM reduction is impactful on a diesel that is designed to run ~2000 rpm, also increasing EGT's. The cooling system cannot handle that much heat input.

-Cheers!
I like the things you said in your post, but this paragraph specifically is interesting to me. I don't think I'm "lugging all the time", but also there is a real difference between your 40s and my 37s, about 8% actually. I assume you recalibrated for tire size like most of us do. I'm going to keep an eye on my rpms all the way home tonight, and see what speed is 2000 RPM in 7th and 8th gears, and then see if that's about my normal cruising speed. Science! Need more data. Will get back to this thread to confirm what we should be able to calculate based on this (among other things):

ZF 8HP75 5.0000 / 3.2000 / 2.1429 / 1.720 / 1.3139 / 1.0000 / 0.8221 / 0.6400 / 3.4560 (Reverse)
Cheers,
 

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I am running 4.56s on 37s but don't have a reference for the stock gears. Getting 19 around town and 24+/- hwy. I like the zip around town and am certain it can get better mileage if I could stay off the pedal. Shell, gear, people plus dog most of the time. Doesn't feel like too much gear. Still haven't hooked up our 5.5k travel trailer (family likes riding in the Ram on long trips). Will see how that goes but I expect it will be fine. Just another data point.
 

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I am running 4.56s on 37s but don't have a reference for the stock gears. Getting 19 around town and 24+/- hwy. I like the zip around town and am certain it can get better mileage if I could stay off the pedal. Shell, gear, people plus dog most of the time. Doesn't feel like too much gear. Still haven't hooked up our 5.5k travel trailer (family likes riding in the Ram on long trips). Will see how that goes but I expect it will be fine. Just another data point.
Are you running a tune? With a GDE and 37s on .73 I struggle to get 19 combined. Has been as low as 16 and that's with a light foot. Rack, Softopper, and about 400# cargo.
 
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PJZ

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Are you running a tune? With a GDE and 37s on .73 I struggle to get 19 combined. Has been as low as 16 and that's with a light foot. Rack, Softopper, and about 400# cargo.
No tune, GDE not available to us in California and dont really want to go with the Banks. Have an AFE drop in filter and use Hotshots Everyday Tx when I fill up, that's it. I have gotten above 20 around town but it was too hard to drive that slow. The 37s are BFGs and really light. Have a second set of wheels and hankook 37s that are bigger all around with lots of poke due to offset. Mileage does drop a small amount but not really significant.

Didnt mention that it feels fast with these gears. My daughter said the same thing when we were getting on the freeway the other day. Never did anyone say my JK ever felt fast :)
 

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I am running 4.56s on 37s but don't have a reference for the stock gears. Getting 19 around town and 24+/- hwy. I like the zip around town and am certain it can get better mileage if I could stay off the pedal. Shell, gear, people plus dog most of the time. Doesn't feel like too much gear. Still haven't hooked up our 5.5k travel trailer (family likes riding in the Ram on long trips). Will see how that goes but I expect it will be fine. Just another data point.
With +/- 24 mpg, you must be in a pretty flat area, yeah? Its hilly where I am at, I’d be surprised if I was able to recover that much regearing. If so, sign me up.
 
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ACAD_Cowboy

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Well since you sound like the right kind of fellow, there is as we know a difference between steady state operation and acceleration.

At a steady state highway cruise of about 65mph the drivetrain only needs a forward energy input of say in 50hp to maintain steady state. This is Fermi math so 50hp works just fine. The road force reaction forces are nominal and winds are favorable.

During acceleration is when we need to realize ALL the power. All say 300 ponies.

Our gearing provides us the mechanical advantages needed to perform this task quickly so we can get out of the pedal and back into steady state thrift. Assuming a fixed power output limit we can numerically increase the gear ratio to accelerate faster with a fixed power input and tire size.

As we increase our tire size we are lowering our effective gear ratios which will then require either increased forward power input or numerically increased gear ratios to correct for.

In our case the diesel has enough excess power capacity to help cover up the lack of mechanical advantage to a degree or at least as much as matters. I suspect the feeling is similar to when I shift from my factoy 32" tires to my 35" tires.

So now we can get into stresses.

The diesel is operating within but towards the far end of cooling parameters and so is likely not going to tolerate a long period of heat loading in stock trim let alone the heat from increased power levels and duration as you would have with lowered gear ratios.

I feel that it would be preferable to add gearing rather than rely on the engine operating at or near peak output continiously. I have owned diesels that existed in a hopped up state and while they did great they didn't do great forever.

Ultimatly the only things that are going to return good fuel economy are lower engine speeds (less fueling cycles per mile) and less throttle input (less fuel volume per cycle). Reducing either or both of these will reduce the amount of acceleration possible and apparent.
 

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With +/- 24 mpg, you must be in a pretty flat area, yeah? Its hilly where I am at, I’d be surprised if I was able to recover that much regearing. If so, sign me up.
Yep, no big climbs around here unless we head north and into the mountains. I also use the adaptive cruise if traffic isn't too heavy. Works well.
 

Escape.idiocracy

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It has nothing to do with you or me. The factory engineered the engine/ drivetrain/ tire size to work optimally together within certain parameters.
so why do they keep a consistent 3.73 gear ratio- even though they offer different size tires between the different models…? 🧐🧐🧐

We are going 4.88’s on 38’s/40’s
Also be getting the gde Canadian/Mexican tune.

engine oil temp is the biggest driver and then crawl ratio.

 
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