Help 3.73 gear ratio, what size tire

[ColoradoDerek]

Good Afternoon all.
I know this question is highly discussed with many opinions across the internet but I figured I'd just ask again anyway and see what YOU guys thought.

I have a Dana 30 front end, Dana 20 transfer case, AMC20 rear end and a T-150 transmission.
My gear ratio is 3.73 and my current tire size is 35". The engine is a AMC 360.

Should I go down to 33" or even 31" tires with this setup? In what scenario would my axles be under too much stress from 35" tires and a 360 motor that they would "snap" or otherwise break? Is that going to be if I'm going hardcore rock climbing day in and day out they will eventually break or is it so touchy that the first rock I try to climb over on a normal trail will break my axles due to my tire size, etc.

I have also heard that with that gearing and my larger tires that going up hills and other climbing will require a lot more "revving" of the engine to keep it going due to the size. How true is that and how much of a difference is a 31" or 33" tire going to be compared to a 35" for "climbing" scenarios?

I want to stay with 35's because I like what they look like but if I need to go down to 33's or even 31's to keep my rig safe and unbroken I can and will.

I can't find any solid information either way on the internet. Half the people say for a 35" tire you need 4.58 ratio minimum or something and others say they have 3.73 and do fine with 35's but I also question whether these people have newer, modern jeeps or old stuff like me. Even more people say 33's are the best for a Dana 30/AMC20 with 3.73 so it's just impossible to find a clear answer, however, right now, before hearing your guys opinions, I'm leaning towards 33's being what I should shoot for.

Since you guys are CJ orientated hopefully I can get some better idea of what I need to do from your opinions.

My goal is to make it so my axles don't snap while I'm driving around. keep in mind, I live in CO and have some badass trails around here so it's possible I could run into a "hardcore" situation just while out "casually" trail riding and I'm wanting to be able to do/try those scenarios without fear my axles will snap.

Thank you!!

EDIT: I Found this old post of mine:
https://jeep-cj.com/community/threads/tires-too-big-for-my-drive-train.66413/#post-566426
which has some information in it to but I figure my question here still stands since now I know all my parts.
After reading that post again though, I'm LESS leaning towards 33's but still kind of am.

-derek

 

[shum8]

Oh, goody - I get to go first and get rocks thrown at me LOL

I am currently running a 304, 33’s, and 3.73’s..... I don’t do heavy rock crawling as this is my DD, but I can offer some insight from the mall crawling aspect (I was jumped all over for having a cheery red mall crawler, but that’s ok - ). When I put it back on the road, I was running 32’s, and I had no complaints. The new tires came in 31’s and 33’s, so I went with the latter. I could feel a difference, but it will still get me a ticket, will still kill me, and in fourth gear will climb a pretty good incline without down shifting. With the added hp of a 360, you should be fine. If you are running ORM 2 piece rear axles, it swap for solids. Copious amounts of skinny pedal will work your u-joints hard, and the driveshaft will complain as well. Those will probly be the weak links....


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[73AMC360]

I had the 304 and 3.73s in my 73. It needed gearing down on the trails. Going downhill was basically scary with my drum brakes. I put the Tera low 3.15-1 kit in my Dana 20. The best of both worlds. fast enough on the freeway. Geared down on the trails. When I put a Dana 44 in the front I needed to make a decision. 4.10 or 3.73. I went with the 4.10s. For running around on the roads I wish I had the 3.73s. For a trail rig the 4.10s along with my 3.15 low range is great. Like you I have a 360. You should have enough torque to run whatever gears you would like.
Sounds like you need a 44 up front so you can have some confidence you wont blow up your front end.

 

[j6824]

Sounds like you need a 44 up front so you can have some confidence you wont blow up your front end.

^This along with beefing your 20 (one-piece shafts, truss, welded tubes) might be the best option for you

 

[Belizean]

How about sticking with the Dana 30 with Warn HD hub conversion if available ?

 

[ColoradoDerek]

If I go down to 33s would I still have to get a new front end or do mods to my rear end, Or even 31s? Which of those is best to avoid any swaps or mods on my front or rear end?


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[73AMC360]

If you watch what you do with yourr right foot you should be okay. You typidcally dont brake anything till you are slipping then a tire grabs.

 

[NP435]

I have 3.73 gears with 33's 258 6 banger with the NP435 trans. All is well .if I had the AMC20 rear and 30 front along with my tru trac diffs I might worry a bit more but if you don't hop under power and burn the tires you might be ok. You sure don't need axle mods for smaller tires.

 

[ColoradoDerek]

Thank you guys, this is all great information.

From what I'm gathering from what you guys have provided (as well as the internet in general) I'm riding on the edge of what my setup is capable of. It sounds like if I just stick to trail riding or other "casual" type rock climbing (rocks on trails, things I can just drive over without too much hassle) then I should be just fine with 35's. If I find myself wanting to do obstacles that might cause me to not move while I'm giving it gas or otherwise is "harder" than just giving it gas and going forward, then that is where I might run into possible issues and even then, only if I start "falling" off the rocks, or otherwise start "slipping" and its the "catch" after the slip that you guys are saying that snaps things.

Knowing all of this I should be able to easily avoid any trails that would potentially hurt me and if I find myself wanting to do them, then either upgrade axles or downgrade tires.

As usual, you guys are amazing, thank you so much!
If I'm not understanding it right based on what I've said above, definitely please correct me
Thanks!!!
Derek Conlon

 

[boney]

The attached spreadsheet might help. It won't give you the strength info you're looking for, but it will tell you your rough calculated speed based on tire size, axle ratio and all that. All theoretical based on the numbers, of course. You're real experience may be different based on all the things that effect the speed of our jeeps (wind resistance, condition of engine, etc.) But it's a start...

It's currently set up for a stock 4.2, T-5, D300. Just change the yellow fields and you're off. If you need to change the transmission ratios change only the blue fields (the rest will change automatically) , and if you need to change the transfer case ratio, just the green one. You don't necessarily need to change the RPM columns, but the Torque and Power highlights will probably be off.

https://1drv.ms/x/s!Ak0FKjfQ1EkOgqV3YUq10w4AWFEaIQ?e=YTrLka

 

[doswell]

Not me, but my cousin is running 33" bf goodrich tires on his CJ5 with 3.73, T-18 6.32. According to him, it works fine for daily driving and some light off-roading.