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Discussion Starter #1
Pretty sure it has been asked before...

With anticipation of towing a camper that is bumping against the limits, I am planning going with gears. Specs of...

-3,500lbs for camper
-6-10 camping trips a year, local this year, eventually stretch to an annual 2,000 mile round trip
-JKUR 3.6 AT, 3", intially on 315/70r17 and planning on 37x12.50r17 eventually
-I'll commute with it a couple of days a week and like speeds of 70mph+ on the highway while preferring camper towing at 60mph max
-I do not anticipate doing "body damage expected" trails, but do want to go on trails just below that spec.

Originally, I approached the gear guy with the idea of 4.88 vs 5.13 and they recommended 4.88s. Read a bunch of threads, then they called back yesterday after review changed that recommendation to 5.13s.

*Anyone see a reason not to go to 5.13s?
*I drove home yesterday forcing the rpms up closer to 3k and the fuel economy improved, perhaps from what I lost with bigger tires?
*Is there a sweet spot for RPMs on the 3.6 motor?
 

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With 35" tires I would rather have 4.88 gears. With 37" tires I would rather have 5.13 gears.
There is a sweet spot with gearing and rpms, where both above and below fuel mileage suffers. When you are geared too tall it affects city mileage more. When you are geared to short (too high a gear number) it affects high speed highway driving more.
 

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5.13 is perfect for 37s. It will be a little deep for 35s, but with towing etc I would do it. It will rock offroad with the 4:1 - you may decide you dont want to even lose that and change your mind on 37s. It will be a hair deep on pavement, but not significantly so. I would be even more inclined to 5.13 if you are a manual. None of this gearing is super deep. And the 3.6 can cruise those rpms quite comfortably. I would personally rather be geared a ratio low (numerically high) than a ratio too high.

My son did a regear contemplating 37s - while he had the 315/70/r17 tires. 2016 Rubicon Hard Rock. He sort of under budgeted and ended up running the 315s for a year or so before finally getting the 37s. He was a manual, and you want to talk about serious crawl offroad on the 315s with the 4:1 (manual has an even deeper first gear than auto)!

I would have a plan B tho. Have a budget to grow the tires before doing the gears - just in case you find it a little deep.

The sweet spot at 70 is around what 37s/5.13 and 35s/4.88 is - that 2700-2800ish range. You're just under peak performance band that 3000ish) so you can get oomph quickly.

Mpg loss is mostly due to larger tires. Proper gearing is getting some of it back because the engine and transmission is operating more efficiently not wasting "work" on gear hunting etc.

I would be very mindful of exceeding tow rating limit tho. Deeper gearing does not change the factory tow rating. Iirc it would be right at 3500 for the 4 door Rubicon.
 

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We ran 4.88/37 on our 4 door for years, regret not going 5.13. We towed a 3500 cargo trailer often (4 times a month) and it was miserable. Now if your trailer frontal area is way below spec then it might be better for you (pop up camper). For us it was so bad we quit using the Jeep for tow duty.

Good luck! :thumb:
 

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Definitely go 5.13.
 
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If 37s are truly right around the corner and in your future, than 5.13s. Couple years away or maybe not at all? 4.88s. 5.13s are too low for a 315/70/17 tire that probably sits at less than 34” mounted.


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When talking tire size and gearing, is everyone referencing the manufacturer stated size or actual measured tire size? Im currently running 35x12.5x17 that measure 33.75 and that is what I set my programmer to with 4.10 gears. Should I change it to 35? Will it only affect how my speedometer reads or does this affect other things?

FYI....My speedometer reads exactly the same speed that a GPS tracker on my phone reads for speed.
 

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When talking tire size and gearing, is everyone referencing the manufacturer stated size or actual measured tire size? Im currently running 35x12.5x17 that measure 33.75 and that is what I set my programmer to with 4.10 gears. Should I change it to 35? Will it only affect how my speedometer reads or does this affect other things?

FYI....My speedometer reads exactly the same speed that a GPS tracker on my phone reads for speed.
Most people go off advertised size when talking about tires. When talking about a "35", there are tires advertised as 35x12.50/17. The 315/70R17 is lumped in with those. As are other tires. A "37" will be sold as a 37x12.50/17 or something similar to that.

Everyone knows (or should know) that tires come off the assembly line shorter than their advertised size. My "35 inch" tire measures right around 33.5" in actual rolling diameter.

Once in a while, someone who's being overly pedantic will raise a stink about how a 35 isn't really a 35.

Thing is, we all know that it isn't a full 35" tall. But we use the convention of calling it a 35 because it's easier than nit picking a detail.

When you go to program your speedometer, make sure you get the actual real world size programmed into the computer so your speedometer is correct. But when talking on a forum, just call it a 35.
 

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Sure am! Gotta do it all proper like
I had a couple close calls due to other drivers being idiots in my work truck pulling a small 6x12 enclosed trailer. If it weren't for the trailer brakes helping, it would have been an accident.
 

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With anticipation of towing a camper ~ 3,500lbs for camper ~ 6-10 camping trips a year ~ eventually stretch to an annual 2,000 mile round trip ~ and planning on 37x12.50r17 eventually
I'll commute with it a couple of days a week and like speeds of 70mph+ on the highway while preferring camper towing at 60mph max
*Anyone see a reason not to go to 5.13s?

As Derf noted, you may want to consider 5.38's.
I ran 35's(33.25") and 5.13's for a couple years, one year of that was commuting daily 130 miles, freeway and interstate... I'd set the cruise on 73mph, which was 3000rpm.
I'm running a heavy 2dr, certainly heavier than a stock 4dr...
Even then, on some longer grades, loaded up with typical recovery gear cooler with food/beverages for a day trip, the rig would loose speed on longer grades (not excessively steep, 5% grade)...
I'm now running 37's(35.25") and miss the previous gearing (smaller tires) enough I'm considering going to the 5.38's... and I don't tow.
5.38's and 35's may have been a bit too much to tolerate at 73mph (3300rpm) but would have been excellent at your towing speed of 60mph (2700rpm).


Oh, this website is invaluable.
http://www.grimmjeeper.com/gears.html


Good luck.
 

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As Derf noted, you may want to consider 5.38's.
I ran 35's(33.25") and 5.13's for a couple years, one year of that was commuting daily 130 miles, freeway and interstate... I'd set the cruise on 73mph, which was 3000rpm.
I'm running a heavy 2dr, certainly heavier than a stock 4dr...
Even then, on some longer grades, loaded up with typical recovery gear cooler with food/beverages for a day trip, the rig would loose speed on longer grades (not excessively steep, 5% grade)...
I'm now running 37's(35.25") and miss the previous gearing (smaller tires) enough I'm considering going to the 5.38's... and I don't tow.
5.38's and 35's may have been a bit too much to tolerate at 73mph (3300rpm) but would have been excellent at your towing speed of 60mph (2700rpm).


Oh, this website is invaluable.
Gear Ratio Calculator


Good luck.
That's an awesome calculator. The guy who built it must be like a rocket scientist or something. ;)
 

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That's an awesome calculator. The guy who built it must be like a rocket scientist or something.

That's really you?
If so, thank you many times over.
many times as in...
1969 F250 390 warmed over gussied up TH727.
1992 Mustang, 3 different engine combos, re-gear each time.
1997 Cherokee... re-geared twice as tires and xfer case changes happened.
2013 JK... noted above.
And 100's of referrals to that site to off-roaders and street/strip crowd as well.



Thanks for sharing that automated spreadsheet/thing with the world.
So easy to navigate, and so helpfull in visualizing it all.
 

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Discussion Starter #17
That's an awesome calculator. The guy who built it must be like a rocket scientist or something. ;)
So funny story...

The installer was guiding me towards 4.88s using the a gear/axle catalog when i posed the original question. He called back a few days later and said that he was speaking with his manager and using they were using the grimmjeeper website and shifted the recommendation to 5.13s.
@derf - Thank you for your website and sharing your knowledge!
 

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I run 4.88 in my 14 JKU auto with 37’s.. I have a few questions for the 5.13, 3.6, auto 37’s crowd

What kind of RPM are you pulling at 80 mph? Cruising at 80 with my 37’s I’m just slightly below 3k RPM..

Second question and the most important one.. those with 5.13’s, what are your RPM’s and what gear are you in when going up a long grade? My JKU downshifts to third at roughly 4500 RPM to maintain 75 mph up grades. I do this for minutes at a time, but couldn’t imagine doing it at 5k plus rpm.
 

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That's really you?
If so, thank you many times over.
many times as in...
1969 F250 390 warmed over gussied up TH727.
1992 Mustang, 3 different engine combos, re-gear each time.
1997 Cherokee... re-geared twice as tires and xfer case changes happened.
2013 JK... noted above.
And 100's of referrals to that site to off-roaders and street/strip crowd as well.



Thanks for sharing that automated spreadsheet/thing with the world.
So easy to navigate, and so helpfull in visualizing it all.
Yep, that's me. Glad so many people find it useful.

So funny story...

The installer was guiding me towards 4.88s using the a gear/axle catalog when i posed the original question. He called back a few days later and said that he was speaking with his manager and using they were using the grimmjeeper website and shifted the recommendation to 5.13s.
That's awesome.

@derf - Thank you for your website and sharing your knowledge!
Happy to do what little I can to help out.
 

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I run 4.88 in my 14 JKU auto with 37’s.. I have a few questions for the 5.13, 3.6, auto 37’s crowd

What kind of RPM are you pulling at 80 mph? Cruising at 80 with my 37’s I’m just slightly below 3k RPM..

Second question and the most important one.. those with 5.13’s, what are your RPM’s and what gear are you in when going up a long grade? My JKU downshifts to third at roughly 4500 RPM to maintain 75 mph up grades. I do this for minutes at a time, but couldn’t imagine doing it at 5k plus rpm.

First, ensure you speedo is properly calibrated... many use a simple cell phone speedo app (myself included).
Your tire size is measured from ground to center of wheel... multiply by 2.
That will get you close, but until you dial in your speedo with a gps/cell, it's really a crap shoot.
Like I said, to get my speedo right (ProCal) for 35's I had a setting of 33.25", and for 37's I'm at 35.25"(or is it 35.75... hmmm, forget, but the 35/33.25 is for certain) tire height. You certainly cannot trust the sidewall, and, most times even ground to center of hub will not get it exact.


I suggest all that, because your speed/rpm/gear math would indicate you are actually running a tire larger than 37"... which is very unlikely if the sidewall says 37.... pointing at an inaccurate speedo, or, maybe 4.56's with a typical near 35" tall 37" tire..



Then go to @derf website... Gear Ratio Calculator
Everything you need/want to know can be extrapolated from there.


You have a Dodge WA580, bottom of the listing under Dodge.
 
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