Why such deep gearing recommendations?

[JMT]

Best rpm for me in the mountains is 2200rpm at 60mph. And I drop a gear, or 2, or 3 for the mountain passes and towing a trailer. Gas mileage goes to hell if your cruise is over 2500rpm.

@greaseorbounce, this comment should pretty much tell you everything you need to know. He’s undergeared, way undergeared. He should be running closer to 3000rpm’s. Then he wouldn’t be dropping gears, which is annoying as hell. And gas mileage does not go to hell. I’ve never gotten better than 15mpg no matter what gearing I had and what tire size.

 

[greaseorbounce]

This has been the discussion I was looking for. Thank you all!

I guess the key takeaway for me here is that my idea of 3000rpm being the top of what I would want to cruise at is unfounded. I guess I need to be more willing to let the 4.0 rev. It's funny because I spend a ton of time on a sport bike turning 8k rpm at cruise and it doesn't bother me, and my daily is an mx5 that happily churns 3300 down the highway. I don't know why I was trying to avoid spinning the tractor engine over 3k. I'll work to recalibrate my ears.

I especially appreciate the comments pointing out the merits of cruising near peak torque. I admit never actually looking at the chart, I had assumed peak torque was in the 2200 range. I was very very wrong there.

I guess when I approach the regear I'll go whole hog and unleash the "high rpm" 4.0.

Thanks again for the insight!

 

[L J]

Now I'm at the point that my next set of tires will be 35s. (Yes everything else is done to support them) I will definitely be regearing at that point, but with 35s all the charts would have me going with 5.13. This is nuts to me! That would have me turning 3000rpm on the interstate. Talk about a screamfest.

I've put over 100,000 miles of driving on my 4.0l engine, 245/75-16 tires and 4.10 gears with a lot of them at 3,000+ rpms. It's not a screamfest, it's within the range that the engine is designed to operate. I've gone to 35" tires and when I regear I'll be going to 5.13s.

Where do these recommendations come from?

decades of collective experience

 

[toximus]

Your ecu can't change the physical gear ratio in your transmission or axles.

The ecu can change how many rpm it idles at and has a learning function. I also cleaned the intake and sensors at one point. I don't know if that can effect the rpm at highway speeds?

 

[GASnBRASS]

The ecu can change how many rpm it idles at and has a learning function. I also cleaned the intake and sensors at one point. I don't know if that can effect the rpm at highway speeds?

Correct, the ecu can adjust the idle rpm, but it cannot change physical gear ratios in the trans or diffs while driving. The only thing it can control while driving that would slightly change rpm is torque converter lockup, which simply eliminates the slippage in the torque converter. But if you've been driving without TC lockup and now it's decided to work then you've had a problem somewhere. If your TJ is a manual then disregard.

 

[tworley]

At 70 I want to say my 35s and 4.56 combo is somewhere around 2600 rpm. I also find myself in 4th far more often since jumping to 35s. 3k feels fine to me cruising down the highway, I cannot wait to get my 5.13s installed.

 

[Sunder]

I have 4.56 gears in my 4.0/3 speed auto. I have 33" tires in winter and 35" in summer.
I definitely prefer the get up and go I feel when I have the 33" tires on.
If I could go back in time, knowing this, I may have gone 4.88.
(I was trying to balance having 33"/35" and concerned about highway speeds.
But now I want my next set of winter tires to be 35"...)

With that in mind, and in addition to all the math previous posters provided, I think deeper gears are also more often recommended because even though a poster says "I'm only go up to 3X" tires"...more often then not, there comes a time when that poster wants to go bigger.

There is just a lot of experience and wisdom given with some of these posters - and they're trying to impart their knowledge, as they've seen these same questions cycle around all the time.

Just like you rarely hear a poster say "I'm so glad I want with the 2.4L instead of the 4.0L", you'll rarely see people complain that they went too deep with their gearing. I mean, it can happen, especially if they actually do go down in tire size - which does happen - but I think happens much less than people wishing they leaned on the deeper side of gearing. Once you feel that "oomph" when you press the gas pedal...it's kinda nice.

 

[greaseorbounce]

As an update: after this wonderful discussion yesterday, I went out and played around a bit today. I just drove down a gear from where I normally would, to really get a feel for how it behaved. I was very surprised how tolerable it was.

My fears of noise were a little unfounded it turns out. Interestingly, it seems that the loudest point is somewhere between 2500 and 3000, and then it seems to quiet back down again. Running down the interstate in 4th this morning at just over 3000 was actually not nearly as bothersome as I thought it would be. Admittedly I had that little voice in the back of my head telling me I needed to shift, but a gag can shut him up.

I'll order a set of 5.13s and slap them in next weekend with some chromoly shafts. The build continues. Back to work.

Maybe I'll even make a post about regearing at home just to rile everyone up. (Yes I have the proper tools, I've done a couple dozen axle rebuilds in my life. Unfortunately nobody ever told my dad it couldn't be done at home, so he taught me the ropes at a young age. It wasn't until much later I learned that "you're not supposed to do that yourself.") Whelp.

Thanks all! I needed that smack in the face to go actually experiment with running 3000rpm down the highway before deciding I didn't like it.

 

[GASnBRASS]

It wasn't until much later I learned that "you're not supposed to do that yourself."

I think the advice to have a driveshop do gear setups is because the majority of us are only doing a gear swap once, maaaaybe twice ever. The cost of tools and lack of an experienced mentor is the major hurdles to doing it at home. Gears are one of the very few things I'll just pay someone with experience to do for me.