Gear ratios and bad advice

Fouledplugs

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awesome thread guys/gals.
i have a 97 2.5l 30rh dana35 dana30 running 35's with stock 3.73 gears. its amazing horrible lol im in the process of gathering parts for a 4.0 ax15 swap next winter and what im gathering is 4.88 gears would be good with the current set up and the 4.0 ax15 set up? or would you suggest 5.13s?
Stock Dana 35? Stock width axles? With 3.37’s, the 4.0 swap will make of little difference.
 

Whispers391

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Stock Dana 35? Stock width axles? With 3.37’s, the 4.0 swap will make of little difference.
yup stock dana 35 and 30. i believe there 3.73's? never checked lol i have a set of dana35 and 30 to truss,sleeve, and put chrome axles in. most of what i do is fire trails, mud, and deepish michigan u.p. sand
 

ChrisTJ98

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Freaking overwhelmed reading this post about gears. Still unsure what the hell to do. 98 TJ 4.0, ax-15, 3.07 gears. I will probably search for a dana 44 to re gear myself but with 31x10.50x15 all terrain bf Goodrich that say they measure an actual 30.5 inches, is 3.73 good enough or should you really go to 4.10's I understand, running at higher rpms is not a bad thing because you do want to run in the middle or slightly higher rpm range of whatever your engine is designed for. It's the same have experienced with outboard motors going with different props. I do want close to maximum mpg as I don't go offload that much anymore. I do live in a very FLAT state.

I guess my question here will 4.10's and this tire size end up eating a lot more into my mpg? Should I just stay with 3.73 gears?
 

wcarson89

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Freaking overwhelmed reading this post about gears. Still unsure what the hell to do. 98 TJ 4.0, ax-15, 3.07 gears. I will probably search for a dana 44 to re gear myself but with 31x10.50x15 all terrain bf Goodrich that say they measure an actual 30.5 inches, is 3.73 good enough or should you really go to 4.10's I understand, running at higher rpms is not a bad thing because you do want to run in the middle or slightly higher rpm range of whatever your engine is designed for. It's the same have experienced with outboard motors going with different props. I do want close to maximum mpg as I don't go offload that much anymore. I do live in a very FLAT state.

I guess my question here will 4.10's and this tire size end up eating a lot more into my mpg? Should I just stay with 3.73 gears?
I will chime in....I have the 3.73 gears stock and recently put on 31x10.5x15 BFG KO2's. I've had them for more than a few months and have probably logged about 12 hours of highway driving on this setup. I think it drives great. On both of my longer road trips I kept the RPM's around 2800 and I was finding myself consistently around 68-69mph. I know that isnt the perfect 70mph @ 2750 rpm's but it's pretty close.

**EDIT** I have the 3-speed auto (32RH) - just noticed you have the manual transmission**
 
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Steel City 06

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Freaking overwhelmed reading this post about gears. Still unsure what the hell to do. 98 TJ 4.0, ax-15, 3.07 gears. I will probably search for a dana 44 to re gear myself but with 31x10.50x15 all terrain bf Goodrich that say they measure an actual 30.5 inches, is 3.73 good enough or should you really go to 4.10's I understand, running at higher rpms is not a bad thing because you do want to run in the middle or slightly higher rpm range of whatever your engine is designed for. It's the same have experienced with outboard motors going with different props. I do want close to maximum mpg as I don't go offload that much anymore. I do live in a very FLAT state.

I guess my question here will 4.10's and this tire size end up eating a lot more into my mpg? Should I just stay with 3.73 gears?
4.10s would be better than 3.73 for sure. In fact, the recommended ratio for 31s and AX-15 is 4.56. I would choose 4.56 personally. 4.10s may be okay in a very flat area.

4.56 would put you at about 2800 RPM at 70 mph. For comparison, a bone stock TJ Rubicon with NSG370 does 2700 at 70 from the factory. So that gets you the closest to the best available factory ratios. It is not revving too high at all.

Check out this thread:

Many users actually saw their MPG go up after a regear going that deep. Better to rev the engine than lug it.
 

Steel City 06

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I will chime in....I have the 3.73 gears stock and recently put on 31x10.5x15 BFG KO2's. I've had them for more than a few months and have probably logged about 12 hours of highway driving on this setup. I think it drives great. On both of my longer road trips I kept the RPM's around 2800 and I was finding myself consistently around 68-69mph. I know that isnt the perfect 70mph @ 2750 rpm's but it's pretty close.
What transmission do you have? I'm guessing 3 speed automatic?

Assuming you have the 3 speed, which has no overdrive, your final gear ratio will be much different than an AX-15, which has a 0.79 OD. So the equivalent gear ratio would be 3.73 ÷ 0.79, or 4.72, which could either be rounded to 4.56 or 4.88.

So if @ChrisTJ98 wants to run the same freeway RPMs as you, he needs 4.56 or 4.88 gears. Of those I would choose 4.56.
 

jjvw

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There is a thread in the how to section with a list of ratios, transmissions and tire sizes. The ax15 and nv3550 are nearly the same OD, so the recommendations are effectively interchangeable. 3k rpm at 75mph is a nice goal to gear for. Your true tire size is the height is the hub x2. Grimmjeeper tells you what you need to know.
 
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ChrisTJ98

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What transmission do you have? I'm guessing 3 speed automatic?

Assuming you have the 3 speed, which has no overdrive, your final gear ratio will be much different than an AX-15, which has a 0.79 OD. So the equivalent gear ratio would be 3.73 ÷ 0.79, or 4.72, which could either be rounded to 4.56 or 4.88.

So if @ChrisTJ98 wants to run the same freeway RPMs as you, he needs 4.56 or 4.88 gears. Of those I would choose 4.56.

Thanks, I believe I'd be comfortable in going with 4.10 gears. Friend has I think a 2000 Jeep TJ exact setup, same tires and motor setup but has D44 3.73 gears and NV 3550 transmission and said his is a slug. I'm headed to Alaska to work the summer so when I come back I'll go take a ride in his and go from there. Thank you all for sharing your knowledge. I just hate pissing money away and not being happy with the decision I have made for why I research the shit out of everything. It's so tuff making that decision because 15-20 years ago people would say 3.73 is perfect. Now I do know that they make a 3.92 for the d44 and 3.91 for the d30 but that odd ball gearing is high as hell in price. Same they make a 4.27 for the d44's as well. I always hated even with factory tires constantly down shifting from 5th to 4th because it couldn't climb highways in Colorado worth a shit when I would take road trips. I'm gonna have to buy a new car soon and trying to decide maybe I should just fix up the old TJ instead buy new, it's a jeep I have had new from high school so a lot of sentimental value to it. Has 147,200 miles so I feel a lot of life left.
 

billiebob

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Cannot quite agree with all the conclusions in post 1. 3.07s with 29s is not a bad ratio.

But I do agree as you get bigger the range of acceptable ratios/rpms narrows. With 35s or 37s the margin is very narrow. With 33s, the range is wider. I'm one of those guys not afraid to down shift, I have a Rubi with 4.10s, I wish I had 3.73s.

Any gear recommendation needs to consider the users needs. Lugging is a function of load more than a function of rpm, there is nothing wrong with 2K rpm and 70mph. EVERY vehicle today with 8,9 speed automatics does this. If you think this is hard on the powertrain, you are the reason why clutches are disappearing, learn to shift.
 

ChrisTJ98

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Cannot quite agree with all the conclusions in post 1. 3.07s with 29s is not a bad ratio.

But I do agree as you get bigger the range of acceptable ratios/rpms narrows. With 35s or 37s the margin is very narrow. With 33s, the range is wider. I'm one of those guys not afraid to down shift, I have a Rubi with 4.10s, I wish I had 3.73s.

Any gear recommendation needs to consider the users needs. Lugging is a function of load more than a function of rpm, there is nothing wrong with 2K rpm and 70mph. EVERY vehicle today with 8,9 speed automatics does this. If you think this is hard on the powertrain, you are the reason why clutches are disappearing, learn to shift.

3.07 and 31's actual 30.5 is absolutely shit no doubt, you get in Mountains of Colorado and will be downshifting to 3rd gear with the 18 wheeler truckers. Been there done that it sucks. 3.73 in my mind may not suck but after what my friend said its possible it still could be shit.
 

JMT

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3.07 and 31's actual 30.5 is absolutely shit no doubt, you get in Mountains of Colorado and will be downshifting to 3rd gear with the 18 wheeler truckers. Been there done that it sucks. 3.73 in my mind may not suck but after what my friend said its possible it still could be shit.
Just go to Grimm Jeeper. It’s very easy to do this. People’s recommendations don’t really matter. Punch the numbers in yourself.
 

ChrisTJ98

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Just go to Grimm Jeeper. It’s very easy to do this. People’s recommendations don’t really matter. Punch the numbers in yourself.
I did that are those numbers accurate, to what my jeep would actually run? I honestly don't know what is the specs for a 1998 TJ 4.0 rpm suppose to run at?
 

JMT

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I did that are those numbers accurate, to what my jeep would actually run? I honestly don't know what is the specs for a 1998 TJ 4.0 rpm suppose to run at?
Yeah, I just compared 4.1 to 4.56. 4.56 all the way. You’ll never be lacking for power 2733 rpm at 70mph. 4.1’s are 2463 rpm at 70. 4.56 is good
 

jjvw

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I did that are those numbers accurate, to what my jeep would actually run? I honestly don't know what is the specs for a 1998 TJ 4.0 rpm suppose to run at?
If you measure your tire size the way I described then Grimmjeeper is very accurate.
 

YourWorstEnemy

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Don't plan to ever go with 33" tires, but I do run 265/75-16, close enough to 32". Per your recommendation above:

4-speed auto (42RLE) used 2003-2006, actually used longer in the JK but I'm not including JK Wranglers.

31's - 4.56, even though the factory Rubicon used 4.10 unfortunately
33's - 4.88
35's - 5.38 (Rubicon's have to change carriers to support this deep of a gear ratio).
37's - forget about it unless you get axles that support 5.89.

Would 4.56 be acceptable to help mitigate the 42RLE engine lugging issue in O/D with 32" tires, or would it need to be 4.88? Most recommendations are based on 33" tires.
Fyi 5.38s are perfectly capable with an LJ on 38s and the 42RLE just in case anyone was wondering...it drives excellent at 70-75 power about the same as it was stock
 

YourWorstEnemy

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Freaking overwhelmed reading this post about gears. Still unsure what the hell to do. 98 TJ 4.0, ax-15, 3.07 gears. I will probably search for a dana 44 to re gear myself but with 31x10.50x15 all terrain bf Goodrich that say they measure an actual 30.5 inches, is 3.73 good enough or should you really go to 4.10's I understand, running at higher rpms is not a bad thing because you do want to run in the middle or slightly higher rpm range of whatever your engine is designed for. It's the same have experienced with outboard motors going with different props. I do want close to maximum mpg as I don't go offload that much anymore. I do live in a very FLAT state.

I guess my question here will 4.10's and this tire size end up eating a lot more into my mpg? Should I just stay with 3.73 gears?
You should just stay with 3.73s....4.10s are not enough difference to go through the gear change...31s and 3.73s will get you better mpgs than 4.10s but the 4.10s will get you a little more power in 5th...although the 4.0 mpg is really not that great to begin with lol...
 

YourWorstEnemy

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awesome thread guys/gals.
i have a 97 2.5l 30rh dana35 dana30 running 35's with stock 3.73 gears. its amazing horrible lol im in the process of gathering parts for a 4.0 ax15 swap next winter and what im gathering is 4.88 gears would be good with the current set up and the 4.0 ax15 set up? or would you suggest 5.13s?
with the 4.0 90 extra hp and the Ax15 4.88 or 5.13 would drive excellent if you want a little better crawl ratio go deeper to 5.13...5.38...if you daily drive it I would probably stick with 4.88s myself