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Gear ratio

Gear ratio
J

jakeh937

Senior Jeeper
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Location
Petaluma, CA
Vehicle(s)
'79 Cj5, 258 with Howell Fuel injection kit, T18 with dana 20 Tcase, dana 30 front with Detroit locker, AMC 20 rear with LSD and one piece shafts, Warn 8k winch, 1" body lift, 2" suspension lift, 33 tires
I'm planning on running my jeep (different then my sog. 79 CJ5 with built AMC 232 i6 , np435 and Dana 20 ). Currently has 3.53 gears in front Dana 30 and rear AMC20 . I plan on running 32-33" tires and will use it mainly on the highway and up in the mountains (more like deer season no rock crawling stuff) what's the better/ best gear ratio?? 3.53 or 3.73. If I go 3.73 what is required? Let's here your input! Thanks!


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I would suggest starting with what you have. There are a lot of charts out there to tell you just what engine rpm you want to drive at. Like this one:
Differential Gear Ratio Chart - Tire Size to Gear Ratio. From there it is a lot of give and take like determining a good highway rpm vs. low end power and using an engine that is good on low end torque.
 
This one is a little hard but here goes...

3.73 rp x 33 tire x 1 to 1 Transmission ratio at 65 mph = 2715 rpm
3.73 x 33 x 1 to 1 at 65 = 2800 rpm

3.54 x 33 x 1 to 1 at 65 = 2577 rpm
3.54 x 32 x 1 to 1 at 65 = 2658 rpm

with your smaller cubic inch motor I think you will want to run the motor a little faster at speed to keep your torque and horse power in range. I have a large V8 so I am trying to keep my highway rpms below 2500. I think you might need to be above 2500 rpm as far as gear swaps the minimum I would do is to get the ring and pinion install bearing kit and swap your gears out. Looking on line it looks lke you do not have to swap either carrier out so thats good.


https://www.ringpinion.com/calculators/Calc_RPM.aspx
 
I love the input!! This is the best place ever. Soooo, some questions. I wanted to build my engine more or less for lower torque ( lugging in the mountains) during my build i swapped in a comp cam and springs to bring the torque curve down lower. First question is what's a good rpm range for that engine? I'm not even sure if I know what red line is on it? Nothing's together as far as a jeep yet just built pieces laying throughout the garage lol.


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As far as driving it, if it's not on the mountain, it'd be more around town/up the coast kinda thing. Sometimes I think I put to much thought into it 🤦🏻*♂️


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Also, what are the benefits/cons to running 3.53 vs 3.73? Besides higher engine rpm? More wear and tear on anything? Or just performance?


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jakeh937 said:
Also, what are the benefits/cons to running 3.53 vs 3.73? Besides higher engine rpm? More wear and tear on anything? Or just performance?


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With the 373 gears you will have a better crawl ratio and the Jeep will respond of the line better but it will not go as fast as the 354 gears


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gert said:
With the 373 gears you will have a better crawl ratio and the Jeep will respond of the line better but it will not go as fast as the 354 gears


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Interesting. I looked up the crawl ratio comparison and it's a difference of about 2:1 so not to much. I'll look into the price to get new r&p for the front and rear axles and see if it's worth it. Worst case I can always change them out later. Now 2 questions for you gert:
1. Is there any added wear and tear through the drivetrain with 354 running 33s?
2. Can I swap r&p from a 74 Dana 30 into a 79 Dana 30 ?


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jakeh937 said:
Interesting. I looked up the crawl ratio comparison and it's a difference of about 2:1 so not to much. I'll look into the price to get new r&p for the front and rear axles and see if it's worth it. Worst case I can always change them out later. Now 2 questions for you gert:
1. Is there any added wear and tear through the drivetrain with 354 running 33s?
2. Can I swap r&p from a 74 Dana 30 into a 79 Dana 30 ?


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I do not think there will be any additional wear with 33s and 3.54

And I believe you can just inspect the ring and pinion throughly for wear and pitting


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gert said:
I do not think there will be any additional wear with 33s and 3.54

And I believe you can just inspect the ring and pinion throughly for wear and pitting


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Awesome. I'll try and do a little more research on swapping over. Seems to me I should be able to.. both dana 30s. Thanks for your help!


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What engine and tranny are you running?
LG
 
Swapping differential gears is not an inexpensive proposition. The gears, bearings, shim kits and seals aren't the expensive part. It's the install that will cost the real money. I realize that expensive for some is not expensive for others. When I did mine (3.73's up to 3.31's for highway performance) it would have cost ~$3000 to do just a simple gear swap. With help form the board it is very doable, but you have to invest some money in a few specialty tools. For most people it's far easier to pay someone else to do the work.

I honestly don't think you will see a great deal of difference between 3.53 and 3.73 gears. I went form 3.73's to 3.31's and saw only a 400 rpm drop at 65mph. My tires are not as large as what you are shooting for, but the change in RPM's is still relatively the same. You are only going up 1 size, 3.53 to 3.73, there will be a difference, but it won't be earth shattering. The question is, will it be worth the expense to you. Maybe yes, maybe no.
 
Hedgehog said:
Swapping differential gears is not an inexpensive proposition. The gears, bearings, shim kits and seals aren't the expensive part. It's the install that will cost the real money. I realize that expensive for some is not expensive for others. When I did mine (3.73's up to 3.31's for highway performance) it would have cost ~$3000 to do just a simple gear swap. With help form the board it is very doable, but you have to invest some money in a few specialty tools. For most people it's far easier to pay someone else to do the work.

I honestly don't think you will see a great deal of difference between 3.53 and 3.73 gears. I went form 3.73's to 3.31's and saw only a 400 rpm drop at 65mph. My tires are not as large as what you are shooting for, but the change in RPM's is still relatively the same. You are only going up 1 size, 3.53 to 3.73, there will be a difference, but it won't be earth shattering. The question is, will it be worth the expense to you. Maybe yes, maybe no.
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After talking with gert on here, I was under the same impression of not really making to much of a difference. It's always something I can swap out later. As far as the tools and stuff I'm a heavy equipment mechanic so I feel confident in accessing the tooling. It'd be more of the how to. But with help from all you guys, I know I could do it! Thanks you


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jakeh937 said:
AMC 232 i6 with comp cam and springs. Np435 mated to Dana 20


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W/33's, and the 1:1 of your 4th gear, you could run 4:10 R&P without much issue. ;)
I say the following, as someone who has setup R&P's for decades.
Watch someone doing this, before you decide or try it on your own. :notworthy:
R&P work, is one of the most precession and detailed jobs you can undertake in automotive work.
Among the tools you need, is a in/lb torque wrench, and a dial indicator that reads to .001". Along with a magnetic base for this indicator. You will also want a ft/lb t'wrench for the ring gear bolts, bearing cap bolts and pinion nut.
Hint: Stay away from 'crush-sleeves'. :eek: Make a solid spacer and use shims.

LG
 
Last edited:
I have 4:10s and 33s in my CJ and the low end is awesome, but the highway cruising leaves a lot to be desired in the form of the engine reving above 3k.
 
Torxhead said:
I have 4:10s and 33s in my CJ and the low end is awesome, but the highway cruising leaves a lot to be desired in the form of the engine reving above 3k.
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What's your hi-way speed at 3K rpm?
Are you sure it really is 3K? Have you confirmed your tach is correct? My OEM tach reads 200-225 rpm high, at 3K rpm indicated.
THX,
LG
 
What's a good rpm for an AMC 232 i6 ? And what's redline?


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