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Old 01-07-2013, 11:09 PM   #1
OregonPlinker's Avatar
Join Date: Dec 2012
Location: Creswell, OR.
Posts: 878
91 YJ ho4.0 auto

3.55's Right now on 31x10.50's. Im gonna go to 33's sometime and Im starting the parts pile asap. Im thinking 4.56"s would be best. I was gonna do 4.10's but I think That will make it drive about like it does now? I could handle a little lower gear I think...

Its not a dd but I do like to drive it to the range, work every now and then and I dont have a trailer for it yet. What do you guys think? Any body running this set up by chance? Right now Im at 2500rpm at 55mph. Or thats what the speedo reads. I dont think its perfectly accurate cause of the 31's.?.

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Old 01-07-2013, 11:33 PM   #2
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Join Date: Jan 2011
Location: El Cajon
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I'd probably not go over 3.73 on 33's and the auto. You want the RPM's below 2500rpm at 65mph, as close to around 2200rpm as possible. 4.56 is too high of a gearing, you don't want to be rolling near 3000rpm at highway speeds, do you? That's where the 2.5L likes it, not so much for the 4.0.

31" tires throw the speedo off by about 11%, so you're doing 2500rpm at around 60mph.


2.5L Genius of some sort.
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Old 01-07-2013, 11:53 PM   #3
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So would you just leave it at 3.55? Or would 3.73 really be worth it? It doesnt seem like much of a change to me...
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Old 01-08-2013, 02:53 AM   #4
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Join Date: Feb 2012
Posts: 101
I'm doing about 2200 rmp at 70 mph with 5" lift and 33" tires with stick gears, it's the 4.0 5 speed. It has no power anymore and can't climb anything. Gears? Or what?
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Old 01-08-2013, 08:42 AM   #5
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Join Date: Jul 2012
Location: south georgia
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I have a 91 with 4.0,auto and 3.55's..it accelerates very good with 33's and will low fly on interstate
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Old 01-08-2013, 02:53 PM   #6
Join Date: Oct 2012
Location: Upstate NY
Posts: 79
I have the 4.0 and I'm running 4.56's on 33's and it does run a tad high on the highway, but it's not something I'm too worried about. The torque is very nice off-road and the power throughout the gears is great. However, I have a manual tranny and I definitely wouldn't recommend this low with an auto, which would spin the engine even higher

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