4.88s or 5.13s? Going from 35s to 37s - Pirate4x4.Com : 4x4 and Off-Road Forum
 
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Old 03-14-2019, 02:23 PM   #1 (permalink)
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4.88s or 5.13s? Going from 35s to 37s

1994 YJ, 4.0, 32RH 3sp, 4.1 Rubicon NV241 Rock-Trac. I usually flat tow sometimes drive 100 miles to the trail. May trailer it down the line. Right now on 35's and 4.10s I'm at 3K at 65mph crawls great with the Roctrac, so I'm a little stumped. Don't want to be at The online calculators aren't helping and seem to be off compared to my actual numbers.

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Old 03-14-2019, 02:49 PM   #2 (permalink)
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What's your desired RPM @ 65 MPH ?

No OD ?

Try this chart.


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Old 03-14-2019, 03:24 PM   #3 (permalink)
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Id probably go 4.56 if you like how it drives now.
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Old 03-14-2019, 03:28 PM   #4 (permalink)
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No OD. 3K at 65mph is ideal now. So my mickey mouse estimate with that chart, 35's with 4.10s should be 2566, so am I correct to say to keep the same RPMs with 37s I want to be between 4.27 and 4.56? Thus 4.88s would be better than 5.13s. See my logic?

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I’d probably go 4.56 if you like how it drives now.
Yeah I guess that would be ideal.....

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Old 03-14-2019, 04:18 PM   #5 (permalink)
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1994 YJ, 4.0, 32RH 3sp, 4.1 Rubicon NV241 Rock-Trac. I usually flat tow sometimes drive 100 miles to the trail. May trailer it down the line. Right now on 35's and 4.10s I'm at 3K at 65mph crawls great with the Roctrac, so I'm a little stumped. Don't want to be at The online calculators aren't helping and seem to be off compared to my actual numbers.
I have the same setup. I run 40's with 5.13's. I can cruise at 65-70mph. My rig is heavy. With 37's I would go with 4.88's. 5.13's would be better but your top end cruise will be lower than 4.88's
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Old 03-14-2019, 04:27 PM   #6 (permalink)
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I wouldnt hesitate to run a 4.0L @ 3k rpm all day, especially with an autotragic. That was peak torque for that engine, depending on which year/model you looked at.

Are you basing the mph off your instruments or GPS? You should be 25-2600 @ 65mph. What axles do you have? That would be my deciding factor between 4.88s and 5.13s. 4.56s would be a waste imho.
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Old 03-14-2019, 04:44 PM   #7 (permalink)
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I wouldnt hesitate to run a 4.0L @ 3k rpm all day, especially with an autotragic. That was peak torque for that engine, depending on which year/model you looked at.

Are you basing the mph off your instruments or GPS? You should be 25-2600 @ 65mph. What axles do you have? That would be my deciding factor between 4.88s and 5.13s. 4.56s would be a waste imho.
I checked out the MPH with a GPS and my speedo is almost dead on. Yeah according to the chart without OD I would be a little bit higher RPM. Sounds like 488s is the ticket.

"These calculations are assuming a manual transmission at 65 MPH with a gear ratio of 1:1. If you drive an automatic your RPMs will be slightly higher, and if you have overdrive your RPMs will be slightly lower."
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Old 03-14-2019, 04:50 PM   #8 (permalink)
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With less than 200rpm difference. It doesnt matter. Go lower crawl better
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Old 03-14-2019, 04:54 PM   #9 (permalink)
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I checked out the MPH with a GPS and my speedo is almost dead on. Yeah according to the chart without OD I would be a little bit higher RPM. Sounds like 488s is the ticket.

"These calculations are assuming a manual transmission at 65 MPH with a gear ratio of 1:1. If you drive an automatic your RPMs will be slightly higher, and if you have overdrive your RPMs will be slightly lower."
Does your 32RH have a lockup TC?
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Old 03-14-2019, 05:06 PM   #10 (permalink)
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Does your 32RH have a lockup TC?
not 100% sure, how do I tell? It may actually be a TF999 but the build sheet states 32RH, I think it may have been replaced. The only reason I say that is because the input shaft on the original 231 T-case was for a TF999.

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Old 03-15-2019, 08:44 AM   #11 (permalink)
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I have a CJ with a similar set up (4.0L/TF999/Dana 300 w/4:1). I was running 35's with 4.56 gears and went to 37's and 4.88s with the latest axle swap. Numerically they are the same but the 37"s are much heavier and took some of the pep away. I wish I had and overdrive and maybe a little deeper gear..... or 100 more HP
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Old 03-18-2019, 09:52 AM   #12 (permalink)
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Running 4.0, 32rh, tons, 40s and 5.13s. So far I haven't driven it much past 50 mph but I think its gonna be pretty good. Sure as hell wish I had OD. I ran 4.88s and 35s previously and running 65 mph it was screaming a little and a little uncomfortable but doable. With 37s you would definitely want at least 4.88s.
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Old 03-18-2019, 10:42 AM   #13 (permalink)
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Running 4.0, 32rh, tons, 40s and 5.13s. So far I haven't driven it much past 50 mph but I think its gonna be pretty good. Sure as hell wish I had OD. I ran 4.88s and 35s previously and running 65 mph it was screaming a little and a little uncomfortable but doable. With 37s you would definitely want at least 4.88s.
Yeah still a little torn. Like @Newb stated not a lot difference between 488 and 513, I do have the roctrac also. But I flat tow 80-90% gonna stew on it some more. Transmission ever goes I'm definitely putting an AW4 in there. Thinking out loud.
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Old 03-18-2019, 11:15 AM   #14 (permalink)
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IMHO the best gear calculator for figuring out RPM's and such.

Gear Ratio Calculator
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Old 03-18-2019, 12:58 PM   #15 (permalink)
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IMHO the best gear calculator for figuring out RPM's and such.

Gear Ratio Calculator
I was gonna disagree with you but I tried something someone suggested and that was measure the wheel at the middle, doublet it and that's your actual tire height. My 35s are closer to 33 actual inches and numbers are lining up on the calculator.
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Old 03-18-2019, 01:11 PM   #16 (permalink)
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I'm in the parts collection phase of my tj build an i have 4.88's in both axles planning for 37's as it seemed to work well in another guy's rig i rode in.
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Old 03-18-2019, 01:13 PM   #17 (permalink)
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I'm in the parts collection phase of my tj build an i have 4.88's in both axles planning for 37's as it seemed to work well in another guy's rig i rode in.
3 speed?
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Old 03-18-2019, 01:36 PM   #18 (permalink)
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My set-up: 4.0/32rh no lock up but I plan to put it on a switch (don't ask why it doesn't work). With 37" pbr's and 4.88s, it feels great. Currently upgrading to 40" kr3's and 5.13's.



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Old 03-18-2019, 08:00 PM   #19 (permalink)
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I'm in the parts collection phase of my tj build an i have 4.88's in both axles planning for 37's as it seemed to work well in another guy's rig i rode in.
5 speed manual, it worked well for him so it seemed like a good way to go for me and with auto's functionally having a lower 1st due to the torque converter and whatnot it seems it would probably work. im just not sure on the overdrive of that trans though so maybe a slightly lower option may help. just trying to share my experiences.

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Old 03-18-2019, 08:16 PM   #20 (permalink)
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I was gonna disagree with you but I tried something someone suggested and that was measure the wheel at the middle, doublet it and that's your actual tire height. My 35s are closer to 33 actual inches and numbers are lining up on the calculator.
Yes you can't just put in the sidewall size to get TRUE numbers. You have to input actual numbers which is why you have to measure from the middle of the wheel to the ground. You can get close numbers by inputting the tire diameter off the manufacture's web site but we all know how not true most of them are.

I have 38x12.5x16.5 TSL SX tires that are 37" tall with no weight on them.
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