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Old 06-03-2014, 04:38 PM   #1
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Need help deciding gear ratio with 4.2l

I am in the process of trying to put 35's on my rig, I have a 4.2l so the rpm range doesn't go as high as a 4.0l. I'm stuck debating between 4.56 and 4.88. I'm worried the I will redline going 65.

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Old 06-03-2014, 08:17 PM   #2
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Rpms are for 4th gear (1:1 ratio) @65mph. The ones in black indicate stock-like performance.

The 4.2L redlines at around 4500 rpms. Regearing to 4.56 on 35" tires would keep you pretty close to the stock rpms at 65 mph. It's far beyond the rpms for peak torque, but closer to that for peak horsepower.

5th gear (overdrive) will drop the rpms a little to give a little better economy on the highway.

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Old 06-03-2014, 08:27 PM   #3
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Rpms are for 4th gear (1:1 ratio) @65mph. The ones in black indicate stock-like performance.

The 4.2L redlines at around 4500 rpms. Regearing to 4.56 on 35" tires would keep you pretty close to the stock rpms at 65 mph. It's far beyond the rpms for peak torque, but closer to that for peak horsepower.

5th gear (overdrive) will drop the rpms a little to give a little better economy on the highway.
I've seen plenty of these charts, but my engine doesn't go above 3500 rpms at all. I want real experience with either of these gear ratios running 35's. Where I live I mostly have city driving and mountain driving so that's why I'm interested in 4.88 gearing. If I don't have to downshift going uphill that would be nice.
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Old 06-03-2014, 09:11 PM   #4
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I don't mean to come off the wrong way, but I think your first course of action is to solve the problem with you motor. If you can't get above 3500 rpms than you will definitely red line on the highway with either gear ratio.
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Old 06-03-2014, 10:12 PM   #5
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I don't mean to come off the wrong way, but I think your first course of action is to solve the problem with you motor. If you can't get above 3500 rpms than you will definitely red line on the highway with either gear ratio.
I'm running 33's now with 4.10 and seem to do fine, I also had 31's before the 33's. The whole not going above 3500 rpms started when I moved to reno's 4500 ft elevation over my old 1000 ft. Haven't been able to tune my vehicle right ever since, part of the reason I'm doing a do it yourself throttle body injection system soon.

I'm also waiting on my new atlas to come in that has a 5:1 low so the 4.56 will probably be just fine, but the 4.88 is not that big of a difference.
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Old 06-03-2014, 10:48 PM   #6
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In that case I would definitely wait to do any gear changes until you have done the new fuel system. You might be quite happy with the results, and if not, well, cross that river when you get to it.
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Old 06-04-2014, 11:30 AM   #7
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So I have today off and I decided to go through my jeep to find the rpm problem, turns out my catalytic converter I put on last year is already fowled. Another reason for fuel injection as I know this happens on my offroad trips when I go to even higher elevations for wheeling and my motor runs extremely rich...
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Old 06-04-2014, 01:36 PM   #8
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Right on! That would not only prevent higher rpms, but also drain power and torque. Throw in a high flow cat and I'd bet you won't be worried about gearing any more.
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Old 06-04-2014, 02:46 PM   #9
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Right on! That would not only prevent higher rpms, but also drain power and torque. Throw in a high flow cat and I'd bet you won't be worried about gearing any more.
It was a high flow cat, and I did pick up another high flow at summit racing this morning. The 4.10 is great for my current 33's, but I'm trying to go to 35's with a 1" body lift and tube fenders within the next month or 2.

I can set up gears all day long so that's no problem, I just can't decide the ratio I want for the 35's. I think I'm leaning towards the 4.88
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Old 06-05-2014, 08:01 AM   #10
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If engine is running right 488s with a 5 speed should be great. JMO
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Old 06-05-2014, 11:00 AM   #11
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If engine is running right 488s with a 5 speed should be great. JMO
That's what I'm thinking too, thanks for the input Nubby
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Old 06-06-2014, 08:35 AM   #12
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I see your running a d44 in the front and you say that you use the jeep for mostly city/around town/ the mountains and 4 wheelin. If it were me with the added resistance of the bigger tires and lift and weight of aftermarket parts along with the real world elements that you just don't get when looking at those bologna gear charts we see all over the magazines and net. I would do 513s. With 513s along with the low end torque of your 4.2 and once you have the TBI installed your jeep will have no issues doing what you ask of it.
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Old 06-06-2014, 12:32 PM   #13
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Your Max HP is 112 @ 3200 rpms and TQ is 210# @ 2000 rpms, so I would want to cruise at about 2500 rpms. Once your in fifth gear your rpms will drop about 300 are so, that said I would want be in the 2,500 to 2,800 range @ 65 in fourth, Max.
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Old 06-06-2014, 01:11 PM   #14
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Your Max HP is 112 @ 3200 rpms and TQ is 210# @ 2000 rpms, so I would want to cruise at about 2500 rpms. Once your in fifth gear your rpms will drop about 300 are so, that said I would want be in the 2,500 to 2,800 range @ 65 in fourth, Max.
My jeep gets the best highway mileage at 2400 rpms, so that falls in with 4.88's, 35's, and 65mph. I hate going over 65 anyway.

My engine with all the current mods dyno tested at about 178hp and 235 lbs/ft tq. I have a highflow exhaust, ported/polished head, bored .030 over, and quite a few of other goodies.

Can't wait to set up all the tbi stuff, just having a hard time finding an adapter for the chevy 4.3l tbi to my Clifford intake. I can have a 4bl to 2bl rochester than the tbi adapter to that, but I want just 1 adapter.
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Old 06-06-2014, 04:46 PM   #15
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I was a Machinist for Exxon, so I would think someone with a Bridgeport type Mill should be able to make one with out any problem. Sorry, I sold my Mill when I Retired and got Divorced at the same time. LOL

She said when I Retired, " I " would have to get rid of my Most Expensive And Useless Things I had around the House. So I told Her, Yes Honey, You Are Right,,, Again!----------SO GOOD BUY!
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Old 06-06-2014, 07:39 PM   #16
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Your Max HP is 112 @ 3200 rpms and TQ is 210# @ 2000 rpms, so I would want to cruise at about 2500 rpms. Once your in fifth gear your rpms will drop about 300 are so, that said I would want be in the 2,500 to 2,800 range @ 65 in fourth, Max.
This look perfect on paper but in reality his jeep will never pull 5th gear @ 65 on anything other than down hill with 488s. Those charts were made back when every 4 wheeler was a V8 with a 3 speed automatic. This is just and only my $.02...I'd love to hear how you made out after the re-gear bud.
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Old 06-06-2014, 07:56 PM   #17
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This look perfect on paper but in reality his jeep will never pull 5th gear @ 65 on anything other than down hill with 488s. Those charts were made back when every 4 wheeler was a V8 with a 3 speed automatic. This is just and only my $.02...I'd love to hear how you made out after the re-gear bud.
I pulled 70 in 5th gear when I had 31's on 4.10's I'm sure I can easily pull 65 with 35's and 4.88's.

Other problem is that my atlas wouldn't even be started by AA until july so I canceled my order for the atlas. My nv241or works just fine anyway. It's just a bit on the long side so I'm considering a teralow and my old np231 with a mega short sye. I want to do a tummy tuck so I feel the nv241 will be too long
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Old 06-06-2014, 10:39 PM   #18
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Right on sounds like you've got it worked out.
I was just pointing out the fact that a lot has changed with jeep since it had 31s.
You've added a lot of extra weight with the new axles and I'm sure you've added the extra wind drag of a lift not to mention the drag and weight of the 35s. Don't forget about the higher elevation.
31s with 410s isn't and apples to apples comparison to 35's with 488s.
I've been down this road my fair share of times.
let us know how it turns out.
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Old 06-06-2014, 11:30 PM   #19
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c5wagner, Maybe I should step out, [I don't have a Dog in this Hunt] I was going by the chart and where My rig fits it, And WOKNROX has a point when I used Charts before I was running V8s and they worked well. I was going with where my rig and it's 4.2 pulled best. They are all defarant, You are not stock buy any means, and You know your rig better than any one does, so Best of Luck, I was just going with my time spent Wheeling, and matching best power at Cruise Speed for my rig not yours, I do not know where you make the most power, TQ meets HP, and as alway, Once angain I stand Corrected.
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Old 06-06-2014, 11:49 PM   #20
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Well, for what it's worth, my 2 cents is this...

I have 4.56 ratio, 35" tires. and at a highway cruise speed of 57 mph, in 5th gear I am turning 2100 rpm. I myself would think that having 4.88 gears would be too deep, if you want to run 65, and your mpg would be very poor because of the higher rpm all the time.

After 2 tanks of gas since the new rear end swap, I am currently holding around 14mpg. But I don't run any 65mph highway either.
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Old 06-07-2014, 12:17 AM   #21
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Well, for what it's worth, my 2 cents is this...

I have 4.56 ratio, 35" tires. and at a highway cruise speed of 57 mph, in 5th gear I am turning 2100 rpm. I myself would think that having 4.88 gears would be too deep, if you want to run 65, and your mpg would be very poor because of the higher rpm all the time.

After 2 tanks of gas since the new rear end swap, I am currently holding around 14mpg. But I don't run any 65mph highway either.
So you're suggesting that I keep my 4.10 gearing with 35's and it will be fine?
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Old 06-07-2014, 12:29 AM   #22
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My gear ratio chart tells me that with your trans and t-case and 35's, with 4.56 gears you will be turning 2077rpm at 65, and with 4.88 you will be turning 2223 at 65.

That is in 5th gear. 4th gear is 2845/4.56, and 3045/4.88 @ 65mph.
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Old 06-07-2014, 12:32 AM   #23
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Go to www.grimmjeeper.com/gears. He has it all figured out with drop down brackets that lets you select the different trans as well as t-case. You fill in the rest and it gives you final calculations at the bottom of the page.

It has been spot on for all the different vehicles that I have ever worked on.
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Old 06-07-2014, 01:01 AM   #24
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I'm running 4.88s, a 4.2, 35s and ax15.

Crawls where it wants and is ok to cruise around town, but 70+ is 26/2700rpm on highway.
15mpg.
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Old 06-07-2014, 01:04 AM   #25
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I'm running 4.88s, a 4.2, 35s and ax15.

Crawls where it wants and is ok to cruise around town, but 70+ is 26/2700rpm on highway.
15mpg.
That sounds pretty bad actually. I have a case with a 4:1 low so I guess I'll try the 4.10's for now and see how I like it. Re gear later if I need to

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