I put 315/70r17 tires on with 3.73's and 6MT and it's fine. WTF?? - Jeep Wrangler Forum
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Old 05-14-2019, 11:58 AM
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I put 315/70r17 tires on with 3.73's and 6MT and it's fine. WTF??

I'm confused. I expected it to be awful. I expected it to be at best mildly tolerable and have already budgeted to get new gears before winter.

But it isn't. It's fine. I expected it to be sluggish off the line. It isn't. It starts off just fine.

In town, I'm normally in 6th gear around 80 km/h and shift to 5th for steep hills. Now I shift to 5th for somewhat less steep hills.

On the highway cruising along at 110 - 120 km/h... it's fine.

I'm honestly astounded.

So now what? I'll have to get out to the trails to see what this has done to my crawl speed but when I was out last weekend before changing the tires I noticed that I spent most of the time in 3rd gear. So if I now use 2nd more... well that's fine. The real difference will be when I'm in 1st. If that's an issue then it will need to be dealt with.

The thing is... I can deal with that by changing to a Rubicon 4:1 transfer case. That should bring my crawl speed to lower than it was before I changed tires, right?

This really changes everything. I was reluctant to put money into my D30 but moving to a Tera44 was way too expensive.

Now it seems I've got the option of going with a 4:1 transfer case if I can find one for a reasonable price and not put a penny into the D30. If I break it some day, I get a Tera44 with a locker.

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Old 05-14-2019, 12:04 PM   #2
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What may be fine for you may not be fine for others.
Your road conditions may be different from others.

If someone lives in a hilly area they are gonna feel the inadequacy of your set up. If someone off roads a lot they by default need a higher ratio.

I put on 315ís KO2ís with 3.73 and spent 200$ and got a PedalCommander to adjust the throttle.

Iíd like mid-400 gear ratio tho


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Old 05-14-2019, 12:05 PM   #3
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3.73 w/ the 6 speed and 35's is tolerable w/ the pentastar. I didn't plan to upgrade gears until i went to several high elevation road trips. Once i added aftermarket weight, then that was the time i felt like i needed a regear. Went 4.56.
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Old 05-14-2019, 12:08 PM   #4
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We all have different expectations in life. You are happy with it, and that is all that matters. 3.73 is a pretty good rounded gear ratio. But you couldn't give me a 3.21......
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Old 05-14-2019, 12:09 PM   #5
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You are absolutely correct in that there really wont notice much decline in performance with 315s/3.73 (in normal daily street driving). I have the same in my JKUR. Where you will notice it is on the highway when you head up to the mountains or if you do much elevation change at all. YOu wont hit 6th gear hardly at all when at highway speed when at elevation (ala 5k or higher). Ask me how I know?!

I would take the money you had allocated to gearing and truss your D30. That will be your weak point. Oh, also get a sector shaft brace!!!!

Im bumping to 4.88s next week due to this.
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Old 05-14-2019, 12:12 PM   #6
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Not sure why you're surprised. Even though a 315 is considered a "35" it measures about 33 3/4 at pressure, well within specs.

I thought our '16 auto with 315/3.73 was acceptable.

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Old 05-14-2019, 12:18 PM   #7
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The Rubi 4:1 transfer case does not change the 4Hi ratio only the 4Lo ratio. 4Hi stays the same as the 2.73:1 transfer case.
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Old 05-14-2019, 12:19 PM   #8
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I'm also running 315's but my stock gears are 4.10 and is fine. If I let go the clutch in 4LO it still crawls without giving it any gas.

Yes, a lot of people with big tires will benefit by re-gearing but 3.73 or 4.10 with manual tranny is not as awful as some would make you believe.

BTW; let's see your Jeep new shoes.
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Old 05-14-2019, 12:19 PM   #9
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315s and 3.73s you can live with................................until you throw on 4.88s,
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Old 05-14-2019, 12:20 PM
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We all have different expectations in life. You are happy with it, and that is all that matters. 3.73 is a pretty good rounded gear ratio. But you couldn't give me a 3.21......
I imagine 3.21's with an auto would be bad. But what do I know?

I get that there may be conditions I'm not facing where the issues would be more apparent. I was just expecting it to at least be a bit bad. Enough that it would take some self-rationalization to justify keeping it this way.

I'm just surprised at how it isn't terrible. The money to change the gears... it feels like I'm not really getting anything for it now.

Is a transfer case going to be a better use for the money? It was never under consideration as the new gears would have also improved my crawl speed, which I've never had complaints about to begin with. But now? What do I do?

If I DO change the gears, is it gonna feel like a rocket ship?
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Old 05-14-2019, 01:06 PM   #11
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315s and 3.73s you can live with................................until you throw on 4.88s,
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Old 05-14-2019, 01:07 PM   #12
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The 4:1 transfer case will only improve your 4L ratio. Gears will improve not only the 4L ratio but also the 2H / 4H ratio.
The manual trans suffers more from gearing than the auto, the torque converter in the auto hides some, plus the auto will obviously automatically down shift for you. With the manual if you have to down shift a lot you have to do it.
For us the stock 32" tires and 3.73 gears were crap. They were too tall off the line, to tall at every hill (requiring a downshift for near every hill) and the Jeep was too slow. Also, off road the crawl ratio was to fast, it would not go slow enough without feathering the clutch (which is bad).
But to each their own. You may drive differently, in a different environment, than we do.
I would suggest you find someone local who has re-geared and see if they will let you try theirs. Ignorance can be bliss, but sometimes you just don't need / want what others need / want.
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Old 05-14-2019, 01:59 PM
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The 4:1 transfer case will only improve your 4L ratio. Gears will improve not only the 4L ratio but also the 2H / 4H ratio.
Yep. I get that. That's why I'm wondering if that might be a better solution on the premise that 4H right now seems legitimately fine.

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For us the stock 32" tires and 3.73 gears were crap. They were too tall off the line, to tall at every hill (requiring a downshift for near every hill) and the Jeep was too slow. Also, off road the crawl ratio was to fast, it would not go slow enough without feathering the clutch (which is bad).
Given that, I expect we do have different expectations. I haven't had any issues with 4L either but might now with the tire change. With the original tires it's been fine. I'm rarely in 1st gear in 4L.

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I would suggest you find someone local who has re-geared and see if they will let you try theirs. Ignorance can be bliss, but sometimes you just don't need / want what others need / want.
This is a good idea. I'll keep an eye out.
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Old 05-14-2019, 02:42 PM   #14
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315s and 3.73s you can live with................................until you throw on 4.88s,


Ainít that the truth I went from 3.73s to 4.88s. No way I could go back. Going up hill passing some one is way to fun now.


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Old 05-14-2019, 03:07 PM   #15
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Ainít that the truth I went from 3.73s to 4.88s. No way I could go back. Going up hill passing some one is way to fun now.


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Mr Jab, mind sharing (if you tracked) what you saw in fuel mileage change between 373 and 488? Just wanting to kind of see what Im going to expect when I make the swap. I know I know, if Im watching fuel economy, Jeep isnt for me, but really Im not watching it. I just want to know if Im going to have to fill up twice or if Ill need rotopax haha.
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Old 05-14-2019, 03:25 PM   #16
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Mr Jab, mind sharing (if you tracked) what you saw in fuel mileage change between 373 and 488? Just wanting to kind of see what Im going to expect when I make the swap. I know I know, if Im watching fuel economy, Jeep isnt for me, but really Im not watching it. I just want to know if Im going to have to fill up twice or if Ill need rotopax haha.


Buy a Prius!

Kidding. Truthfully Iím not sure the impact. I never paid too much attention and lately I have been traveling the interstate for work so it is skewing my numbers.

To be honest I think it went up 1 mpg since I donít have to work so hard going up hills where I live.

Maybe someone else can chime and see if that makes sense.

What I do know is my wife handed me my credit card statement and said she thinks itís wrong. Too many Sheetz (gas station) charges. I laughed looked at it. Unfortunately it looked accurate!


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Old 05-14-2019, 03:40 PM   #17
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<snip> I'm rarely in 1st gear in 4L.
<snip>
Then it might work out for you just fine... I'm almost never in anything but 1st gear when in 4 lo with my auto.

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Mr Jab, mind sharing (if you tracked) what you saw in fuel mileage change between 373 and 488? Just wanting to kind of see what Im going to expect when I make the swap. I know I know, if Im watching fuel economy, Jeep isnt for me, but really Im not watching it. I just want to know if Im going to have to fill up twice or if Ill need rotopax haha.
When I went to the 4.56, my in town mileage seemed to improve slightly and my freeway mileage got a decent amount worse.
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Old 05-14-2019, 03:50 PM   #18
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If you have an auto, 3.73's and 315-70-17's and you want better MPG, slow down to <65. If you want more power on the freeway use 4th. 4.56's are probably better but,,, $$$.
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Old 05-14-2019, 05:42 PM   #19
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I imagine 3.21's with an auto would be bad. But what do I know?

I get that there may be conditions I'm not facing where the issues would be more apparent. I was just expecting it to at least be a bit bad. Enough that it would take some self-rationalization to justify keeping it this way.

I'm just surprised at how it isn't terrible. The money to change the gears... it feels like I'm not really getting anything for it now.

Is a transfer case going to be a better use for the money? It was never under consideration as the new gears would have also improved my crawl speed, which I've never had complaints about to begin with. But now? What do I do?

If I DO change the gears, is it gonna feel like a rocket ship?

Yes; yes it will.
3.21s auto to 4.56s was night and day. Trail and as a DD.
Unless you're doing a ton of wheeling I'd throw your money at a re-gear way before the Tcase. Rock crawling would be a lot nicer/smoother with a Tcase sure- but the last one I saw was $1300. For that I'd 100% spend your $$ on a re-gear and enjoy my drive on the daily instead. If 3.73s don't bother you then leave it. But the Tcase has limited benefits unless your rig is strictly for trail use or a LOT of trail use.
I've rock crawled on my D30 S & G'ed for 5 years. Lots of peeps will say "don't throw your money at a D30" but I know plenty of people hard core rock crawling on D30s. The kicker is what you can reasonably get it done for in your area. But still it's $1000+ less than a replacement axle for most.
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Old 05-14-2019, 05:48 PM   #20
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Mr Jab, mind sharing (if you tracked) what you saw in fuel mileage change between 373 and 488? Just wanting to kind of see what Im going to expect when I make the swap. I know I know, if Im watching fuel economy, Jeep isnt for me, but really Im not watching it. I just want to know if Im going to have to fill up twice or if Ill need rotopax haha.
Rotopax.
There is some myth that you'll get better gas mileage with a (numerically higher) gear. All myths start with truth...and some people DO get better. But it's a rarity. I had this illusion that once I stopped stomping on the gas with 3.21s...and trying to keep it out of the 3.21 lug zone that the MPG tooth fairy would come and visit. She has flown past me....laughing the whole way.
I would realistically assume 15 MPG with 4.88s and 35s. Might be more. Might be less. But don't spend $1000 on re-gear and what, $1200 on tires....if that figure crushes your soul. And your wallet.
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Old 05-14-2019, 06:07 PM   #21
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I'm confused. I expected it to be awful. I expected it to be at best mildly tolerable and have already budgeted to get new gears before winter.

But it isn't. It's fine. I expected it to be sluggish off the line. It isn't. It starts off just fine.

In town, I'm normally in 6th gear around 80 km/h and shift to 5th for steep hills. Now I shift to 5th for somewhat less steep hills.

On the highway cruising along at 110 - 120 km/h... it's fine.

I'm honestly astounded.

So now what? I'll have to get out to the trails to see what this has done to my crawl speed but when I was out last weekend before changing the tires I noticed that I spent most of the time in 3rd gear. So if I now use 2nd more... well that's fine. The real difference will be when I'm in 1st. If that's an issue then it will need to be dealt with.

The thing is... I can deal with that by changing to a Rubicon 4:1 transfer case. That should bring my crawl speed to lower than it was before I changed tires, right?

This really changes everything. I was reluctant to put money into my D30 but moving to a Tera44 was way too expensive.

Now it seems I've got the option of going with a 4:1 transfer case if I can find one for a reasonable price and not put a penny into the D30. If I break it some day, I get a Tera44 with a locker.
With the manual transmission you probably instinctively adjusted your shifting and clutch.
You would fill it with auto for sure, it tends to keep the rpm in the lower green zone and it sucks on the hills. It is not about the ratio its about the software in the damn gear box. When I drive it in manual mode everything is fine I feel like there is enough momentum for larger wheels even with 3.21.
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Old 05-14-2019, 06:08 PM   #22
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I'm confused. I expected it to be awful. I expected it to be at best mildly tolerable and have already budgeted to get new gears before winter.

But it isn't. It's fine. I expected it to be sluggish off the line. It isn't. It starts off just fine.

In town, I'm normally in 6th gear around 80 km/h and shift to 5th for steep hills. Now I shift to 5th for somewhat less steep hills.

On the highway cruising along at 110 - 120 km/h... it's fine.

I'm honestly astounded.

So now what? I'll have to get out to the trails to see what this has done to my crawl speed but when I was out last weekend before changing the tires I noticed that I spent most of the time in 3rd gear. So if I now use 2nd more... well that's fine. The real difference will be when I'm in 1st. If that's an issue then it will need to be dealt with.

The thing is... I can deal with that by changing to a Rubicon 4:1 transfer case. That should bring my crawl speed to lower than it was before I changed tires, right?

This really changes everything. I was reluctant to put money into my D30 but moving to a Tera44 was way too expensive.

Now it seems I've got the option of going with a 4:1 transfer case if I can find one for a reasonable price and not put a penny into the D30. If I break it some day, I get a Tera44 with a locker.
With the manual transmission you probably instinctively adjusted your shifting and clutch.
You would notice it with auto trans for sure, it tends to keep the rpm in the lower green zone and it sucks on the hills. It is not about the ratio its about the software in the damn gear box. When I drive it in manual mode everything is fine I feel like there is enough momentum for 1-2" larger wheels even with 3.21. When back to auto-mode its great only upshifting and then it stays in high gear regardless. I can make her to downshift but I have to apply a lot of gas more then I would want to. With the manual In the same situation I would downshift in advance keeping the engine at optimal rpm.
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Old 05-14-2019, 06:25 PM   #23
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Mr Jab, mind sharing (if you tracked) what you saw in fuel mileage change between 373 and 488? Just wanting to kind of see what Im going to expect when I make the swap. I know I know, if Im watching fuel economy, Jeep isnt for me, but really Im not watching it. I just want to know if Im going to have to fill up twice or if Ill need rotopax haha.
We went from 3.73 gears to 4.88 gears after installing bigger tires. The bigger tires killed the fuel mileage, from around 18 mpg down to around 15 mpg. The 4.88 gears got the mileage back up to between 16 and 17 mpg, with 16 being more typical on highways and higher speed roads while 17 is common at or below 65 to 70 mph.
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Old 05-14-2019, 08:34 PM   #24
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Originally Posted by Wild Weasel View Post
I'm confused. I expected it to be awful. I expected it to be at best mildly tolerable and have already budgeted to get new gears before winter.

But it isn't. It's fine. I expected it to be sluggish off the line. It isn't. It starts off just fine.

In town, I'm normally in 6th gear around 80 km/h and shift to 5th for steep hills. Now I shift to 5th for somewhat less steep hills.

On the highway cruising along at 110 - 120 km/h... it's fine.

I'm honestly astounded.

So now what? I'll have to get out to the trails to see what this has done to my crawl speed but when I was out last weekend before changing the tires I noticed that I spent most of the time in 3rd gear. So if I now use 2nd more... well that's fine. The real difference will be when I'm in 1st. If that's an issue then it will need to be dealt with.

The thing is... I can deal with that by changing to a Rubicon 4:1 transfer case. That should bring my crawl speed to lower than it was before I changed tires, right?

This really changes everything. I was reluctant to put money into my D30 but moving to a Tera44 was way too expensive.

Now it seems I've got the option of going with a 4:1 transfer case if I can find one for a reasonable price and not put a penny into the D30. If I break it some day, I get a Tera44 with a locker.

I have 315's and 3.73 as well. No problems at all. You will however notice your mileage taking a nose dive with the wider wheels. If I'm lucky I get 19L/100km. In the Winter it goes up to 23
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Old 05-14-2019, 08:46 PM   #25
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I have 315 (34.25 actual) with factory 4.10 and itís the perfect combo for me.


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Old 05-14-2019, 08:50 PM   #26
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Re-gear=huge difference

I had 3.21 gears and thought it was fine,until I lived with it for a while with 34" and 35" tires. I noticed all the downshifting on hills and or headwinds,it was not good and didn't feel good for the trans. I went with 4.56 gears,and it made one hell of a difference, no more downshifting on hilly roads,great crawling off road in 4-Lo and still get 12'sL per 100km range on the highway at the speed limit or a touch higher,here in Canada the speed limits are 80 to 90 KPH secondary two lane highways,and 100km's on the big multi lane highways. The skinnier 35X11.5 Grapplers are helping in the KPL department.although I'm not concerned with that too much...cause it is a Jeep...Smiles per Gallon not Miles Per Gallon.

Bottom line..the Jeep runs the way it's supposed to run with proper gearing.
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Old 05-15-2019, 06:48 AM
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Then it might work out for you just fine... I'm almost never in anything but 1st gear when in 4 lo with my auto.

When I went to the 4.56, my in town mileage seemed to improve slightly and my freeway mileage got a decent amount worse.
I've done some really slow rock crawling in the past and will do some again in the future but the trails that are closer to me tend to be more dirt and a bit faster. So I'm rarely in 1st gear for any distance.

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Originally Posted by YnotAJeep View Post
If you have an auto, 3.73's and 315-70-17's and you want better MPG, slow down to <65. If you want more power on the freeway use 4th. 4.56's are probably better but,,, $$$.
This is true no matter what gears you have. I tend to do around 110 km/h on the highway for long trips. It's fast enough to not be holding up traffic and not get passed by 18 wheelers but I'm definitely one of the slower vehicles out there.

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Originally Posted by JeepHerz View Post
Rotopax.
Might be less. But don't spend $1000 on re-gear and what, $1200 on tires....if that figure crushes your soul. And your wallet.
Man. I need to move to where you are! My wheels and tires were nearly 5 grand and I budgeted $3k for the re-gear!!

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Originally Posted by Bob Sanders View Post
I have 315's and 3.73 as well. No problems at all. You will however notice your mileage taking a nose dive with the wider wheels. If I'm lucky I get 19L/100km. In the Winter it goes up to 23
Yeah. I'm expecting this. It is what it is.

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Originally Posted by Jeeperitis View Post
Bottom line..the Jeep runs the way it's supposed to run with proper gearing.
The more I drive, the more I'm realizing that it's definitely compromised over what it was. I noticed last night that I'm in 3rd and 4th gear way more than I used to be and that making a corner without slowing down too much now still needs 2nd gear instead of 3rd as it used to.

None of this is unbearable, but the little issues are definitely starting to show themselves. I've stopped even trying to use 6th gear around town.

Most of my everyday driving is in the city so if the re-gear will help city mileage but hurt highway mileage, that will probably be a net gain for me.

I've decided to go ahead and do it. I now understand why people say it's fine as it is but hopefully I'll end up in the camp of people who say it might be ok but that 4.56's are way better.

I planned to do this all properly so there's no point in "just living with it" now. Why have it "just ok" when it could be as good or better than it was?

I just really hope I don't feel it was a waste in the end.
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Old 05-15-2019, 07:29 AM   #28
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Originally Posted by Wild Weasel View Post

I just really hope I don't feel it was a waste in the end.
I can't speak for you, but for us re-gearing was the best money we have spent on our Jeep and we have spent a lot. The stock gearing was less than great. With bigger tires it was worse. Re-gearing didn't just get it back to where it was for us, we went further with the gearing so now our Jeep has more power at the tires than it ever had. Even though the tires are a lot bigger and heavier it still accelerates harder than it did with the stock tires and gears. You don't need to go to 4.88 gears like we did, you may prefer 4.56 gears. But that is up to you. You can keep it like it is, nothing will break because you didn't re-gear. But if you re-gear you may find your Jeep is more fun to drive and you may even find it gets slightly better fuel mileage overall.
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Old 05-15-2019, 07:51 AM
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Ultimately, fun to drive is what I'm going for.

The plan all along was to go with 4.56 gears. I'm not re-considering that. 4.88's aren't in the picture as I'm never going with bigger tires than I've now got and I need it to be useful for road trips.
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Old 05-15-2019, 09:02 AM   #30
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Originally Posted by Wild Weasel View Post
Ultimately, fun to drive is what I'm going for.

The plan all along was to go with 4.56 gears. I'm not re-considering that. 4.88's aren't in the picture as I'm never going with bigger tires than I've now got and I need it to be useful for road trips.
That sounds like a plan. And 4.56 gears may be the perfect gears for you. We have 4.88 gears and take road trips from Maryland to Moab Utah and back, no problems. That is around 6,000 miles round trip, much of it at 80+ mph out on the highway. The difference in rpms at 70 mph between 4.56 gears and 4.88 gears is only a couple hundred rpm. It really isn't a big difference. But 4.56 gears may be perfect for you. All I can say is so far I have only heard one person complain they went too far with gearing when they re-geared. Most people are either happy with the new gears or wish they went a step further.

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