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Discussion Starter #1
I was going down the freeway at about 70 and wondered what it will be like with 4.56 gears, I have 3.73's. So I put it in 4th gear and it looks like the revs go right to where they would be with a 4.56 re gear. Doing some math says it is really close. So, it performs well around town and for real 4x4 I do have the 4:1 TC for low range. If I am going to re gear for better highway performance, why not just take it back to 4th and call it good. The 3.73's seem to be just fine and the $2K will come in handy other places. BTW 4th is a 1:1 gear so it could even run cooler. 5th would be a great down hill gear for coasting. Is there a flaw in my thinking to really need the 4.56's. I do know that a 5 speed that uses all 5 is ideal.
 

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I have a similar setup (35 BFG KM2 and 3.73s on an Auto Rubicon). For me the issue is the driveability in ANY gear at lower RPMs (1000-2000rpm). I feel that there is just zero acceleration. For example, cruising down a street at 35mph in 4th or even 3th and i come up to a hill or need to accelerate. My Jeep HAS to downshift if i want any sort of acceleration. On the freeway cruising in 4th is fine, but when i get to hills it still needs to downshift to 3rd or even 2nd. I think the general driveability is just crap. Gears (i will be going with 4.88s) will help out significantly. Your general though of, just down shift is somewhat correct, but remember that the gears in the diffs will affect all gears in the transmission. Each gear will have a lower final drive ratio which helps across the board of all speeds and rpms.

Suggestion: find someone local who will let you drive a regeared rig. I think it would change your mind.
 

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In a single gear comparison, yes you can get really close. But there is so much more to it than top gear cruising. The gaps in gearing change as you go through the gears, so you are giving up efficiency. And most importantly, is the first gear... It's much higher, much less efficient and would be sluggish in comparison. Hard on the clutch too.

But, it's up to you what is acceptable. I am still on 3.73, mainly because I could never decide on what the final build was supposed to be. I don't mind changing height, or adding tube fenders, etc. Tires are not a big deal, buy bigger, sell the others, etc... But gears is a $1200 bill and if you need to change, it's another $1200. And if you want to add a locker or LSD, another $1200 if you don't do it at the same time as gears, etc.

But I'm really close to making the call to stay at 35's on the 4dr which means I will go 4.56. I also plan to add a 2dr Rubi to the family in a year or 2 and it starts all over... 37's and 5.13???
 

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This is a common misconception. While you are at the same rpms the 4.56 gears are putting more power to the ground. The engine produces about 250 ft lbs of torque from about 2000 rpms through about 3600 rpms. It is what our gears do with that more that makes the difference. Here is the math.

3.73

5th gear

3.73 = 2081 rpms = 250 ft lbs

3.73 x 250 = 932.5 x 2081 / 5252 = 369 hp

4th gear

3.73 = 2507 rpms = 250 ft lbs

3.73 x 250 = 932.5 x 2507 / 5252 = 445 hp

4.56 Gears

5th gear

4.56 = 2544 rpms = 250 ft lbs

4.56 x 250 = 1140 x 2544 / 5252 = 552 hp

As you can see 4.56 gear even in 5th still puts more power to the ground than 3.73 in 4th.
 

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This is a common misconception. While you are at the same rpms the 4.56 gears are putting more power to the ground.

As you can see 4.56 gear even in 5th still puts more power to the ground than 3.73 in 4th.

i can attest to that statement. when i was running 3.73 with 37's, i had to stay in 4th gear to have any power. now with the 4.88's, i can use 5th in places that i couldnt use 4th before.
 

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Discussion Starter #6
OK ya got me. Now I'll have to spend those bucks someday. I know that numbers never lie,,LOL. With the 4.56's giving me 552 HP in 5th I should be able to smoke the tires and hit 140MPH easy.
 

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I'm still scratching my head. I have a stock Rubi/Auto. It is equipped with 3.73 gears and stock tires at 255/75/17s. I want to go to bigger tires. My target currently is 315/70/17s.

When I do the arithmetic that is roughly a 7.2% increase in tire diameter.

I'm OK with the way the Jeep runs right now. I can cruise at 70 on the freeway at about 2300 RPM and get decent mileage (for a vehicle with the aerodynamics of a brick). Off road there is plenty of power and if I go to 4-LO, even more.

Seems to me that if I go to 4.10s I'm still going to see some improvement (3.73 x 1.072 = 4.00 (rounded)) without having to spin the engine faster on the freeway. Thus it seems that 4.56s are overkill. Am I missing something?
 

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OK ya got me. Now I'll have to spend those bucks someday. I know that numbers never lie,,LOL. With the 4.56's giving me 552 HP in 5th I should be able to smoke the tires and hit 140MPH easy.
You do know that HP is just a made up number correct? It's torque x rpm / 5252. It can be applied to anything to use for comparison purposes. All it really shows is the amount of work being done.
 

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I'm still scratching my head. I have a stock Rubi/Auto. It is equipped with 3.73 gears and stock tires at 255/75/17s. I want to go to bigger tires. My target currently is 315/70/17s.

When I do the arithmetic that is roughly a 7.2% increase in tire diameter.

I'm OK with the way the Jeep runs right now. I can cruise at 70 on the freeway at about 2300 RPM and get decent mileage (for a vehicle with the aerodynamics of a brick). Off road there is plenty of power and if I go to 4-LO, even more.

Seems to me that if I go to 4.10s I'm still going to see some improvement (3.73 x 1.072 = 4.00 (rounded)) without having to spin the engine faster on the freeway. Thus it seems that 4.56s are overkill. Am I missing something?
Nope not missing anything. 4.10's and 35 will give you about the same power you have now with 32 inch tires and 3.73. But many of us find the power of our Jeeps to be a bit lacking. I started with 32's and 4.10, moved up to 35's and finally regeared to 4.88. Even though I turn big RPM's at 70 mph, around 2800 I am still getting the same MPG I was when running 4.10's and 35's. But I am able to maintain speed even better than when I was stock. But if you are happy now then 4.10's may be your ticket.
 

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I'm still scratching my head. I have a stock Rubi/Auto. It is equipped with 3.73 gears and stock tires at 255/75/17s. I want to go to bigger tires. My target currently is 315/70/17s.

When I do the arithmetic that is roughly a 7.2% increase in tire diameter.

I'm OK with the way the Jeep runs right now. I can cruise at 70 on the freeway at about 2300 RPM and get decent mileage (for a vehicle with the aerodynamics of a brick). Off road there is plenty of power and if I go to 4-LO, even more.

Seems to me that if I go to 4.10s I'm still going to see some improvement (3.73 x 1.072 = 4.00 (rounded)) without having to spin the engine faster on the freeway. Thus it seems that 4.56s are overkill. Am I missing something?
I had 3.73s and also calculated to 4.10s. My terrain around is here have 1000ft alt changes plus I tow. That is why I stepped up to the 4.56s.
 

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I'm still scratching my head. I have a stock Rubi/Auto. It is equipped with 3.73 gears and stock tires at 255/75/17s. I want to go to bigger tires. My target currently is 315/70/17s.

When I do the arithmetic that is roughly a 7.2% increase in tire diameter.

I'm OK with the way the Jeep runs right now. I can cruise at 70 on the freeway at about 2300 RPM and get decent mileage (for a vehicle with the aerodynamics of a brick). Off road there is plenty of power and if I go to 4-LO, even more.

Seems to me that if I go to 4.10s I'm still going to see some improvement (3.73 x 1.072 = 4.00 (rounded)) without having to spin the engine faster on the freeway. Thus it seems that 4.56s are overkill. Am I missing something?
You are trying to match up to your 3.73 with the larger tires. Most of us are trying to match up to the factory 4.10 for the most part. The consensus has definitely been 4.56 for 35's and some people even go to 4.88.
 

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Discussion Starter #13
Spinning close to 3K rpm for hours is no big deal. My 70 Ford Bronco did 3000rpm at 70 and I did that for hours on end. A 302 built in 70. The 3.6 is built to do more than that, longer. Would be nice to see the JK faster 0-60. Now my 8000 pound stock Ram diesel can easily walk away from my JK. But, the JK making a hard right and get on it and it feels like the rear is almost going to brake loose. With 4.56's it better.
 
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