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For those of us with 2012+ JK/JKU’s
There are so many threads asking the question: What gear ratio should I upgrade to? There are many knowledgeable people on the forum who offer good advice, but unless the person seeking the advice can relate it back to his own setup, it can be quite confusing. So before you start asking the random question: Should I re-gear to 4.88's? Understand first, it all depends on what transmission you have, and what tire size, and what you're going to do with your vehicle.

The great thing about internet forums is that you get a lot of information from people who’ve already “been there, done that” and can shed light on your question. Unfortunately, there’s also quite a bit of knee jerk responses that add to the confusion. ”4.56!” …”4.88, anything else is a waste!” …”5.13’s, best setup period!” Sound familiar? All of these suggestions can be valid, but it always comes back to: Where are you starting from and what are you trying to accomplish? For most people who are asking the question,

I think it comes down to: How do I offset the performance loss experienced by adding larger tires to my rig? And, how not to go overboard and adversely affect on road driving performance and MPG. This is where the forum responses need some help in getting folks on the same page. Suggesting 4.56’s to someone with an automatic transmission might be ideal for a given situation, but would have a much different result for a manual transmission rig.

The purpose of this thread is to try and layout some basic guidelines for the guy who depends on his vehicle as a daily driver, who just wants to get back some power loss, without affecting your JKU’s driveability. I’m not addressing the crowd that’s building a rock-crawling monster. You’re on your own! Also, I am not an expert. I want to seek input from others who can shed some light on this, based on their own results. But I think it would be a great help to just layout some basics. So if you start with gear ratio x and tire size y, ... How do I get back to x after I change y?

So this is what I want to know... Am I looking at these charts correctly?
I've studied the gear chart until the colors blur together, and it's fine but I want a little more than an eyeball equation. I really like the Grimmjeeper's gear ratio calculator Gear Ratio Calculator


Basic guidelines for equating to stock. (*based on 255/75/17 stock tire size for 2012+ models) In this example I’m using 315/70/17 for y, as that seems to be the most popular tire size, and has a significant impact on real world driving, throttle response, and MPG.

Manual Transmissions – Stock equivalent
3.73’s stock = 4.10 upgrade (315/70/17) gives you <+100 RPM variance compared to stock in all gears.
4.10’s stock = 4.56 upgrade (315/70/17) gives you <+100 RPM variance compared to stock in all gears.
Automatic Transmissions – Stock equivalent
3.73’s stock = 4.10’s (315/70/17) gives you <+100 RPM variance compared to stock in all gears.
4.10’s stock = 4.56’s (315/70/17) gives you <+100 RPM variance compared to stock in all gears.

But here’s where it gets interesting! Using lower gear ratios that are not stock equivalent. Obviously, these are examples where you probably would not want to be if you’re mostly on road, but would be more useful if you’re building more to an off-road or crawler spec.
Manual 3.73 to 4.56 - The 3.73 – 5th gear is about the equivalent of 4.56 - 6th gear
Manual 4.10 to 5.13 - The 4.10 – 5th gear is about the equivalent of 5.13 - 6th gear

Auto 3.73 to 4.56 - The 3.73 – 4th gear is about the equivalent of 4.56 - 5th gear
Auto 4.10 to 4.88 - The 4.10 – 4th gear is about the equivalent of 4.88 - 5th gear

And finally, just an interesting little tidbit when you start cross-referencing manual transmission gear ratios to automatic transmission gear ratios, (tire size being equal
MT-3.73 vs. Auto-4.56 has almost identical RPM’s across the gears 1-5, but obviously you still have OD with the manny! This makes me feel better as I have the equivalent of an auto w/ 4.56's, but upgrading to 4.10's might just be nirvana.... and cheaper than getting a Rubicon!

So? Is this a good guideline or am I missing it?
 

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Good points and i'm not an expert. As I have decided to re-regear I've recently gone through the same frustration and come to many similar conclusions. Nevertheless, when you add in different tire sizes, it gets even more interesting and when you factor in completely subjective things like driveability, or road feel, off road use to road use ratio..GAH!

That being said, my conclusion with my 2012 2 door rubicon manual transmission is that I should do 4.88 with 37s. If I had an auto, it would be 5.13. Completely subjective based on hours and hours of reading and I'm still not 100% that I'm correct!

Also, if you want cheap 4.10s when I'm done in 2-3 weeks, just PM!
 

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Thanks for linking my calculator. :thumb:

One good place to start for a "gears 101" type explanation is the page I wrote up on regearing basics. It covers in somewhat plain English why you want to change your gears when you change your tires.

The long and the short of it is that you should start with your original gears and tire size and apply a simple formula you can plug into a calculator:

old gears x new tire size ÷ old tire size = new gears

Round to the nearest gear size that you can find for your axles. Because of added weight and rolling resistance of larger tires, it's best to round up.

To get better off-the line performance and better crawling, go up from there.


My own personal experience is going from 3.21 gears and 32" tires to 35" tires. I started knowing that the 3.21 gears weren't right for the stock tires and that I wanted more off-the-line performance than staying stock. But I started with what I had and the size of the tires I wanted to go to.

3.21 x 35 ÷ 32 = 3.51

Round up and you get 3.73's. Trouble is, I'd still have the same problem I had before. Not enough power off the line and overdrive dropped RPMs too far so 6th gear was essentially useless. I knew I wanted to step up from there but at this point I know where my starting point is for working with that size tire. Researching the axles in the Jeep, I know my options were 4.10, 4.56, 4.88, 5.13, and maybe a couple more.

From there, I dropped the numbers into my ratio calculator to see where the RPMs ended up on the highway. Initially I was split between 4.10's and 4.56. I think I would have been happy with 4.10's. They would have been "enough". But the 4.56 gears were a little easier to find (and maybe cheaper too, I forget). They're plenty for my daily driving and moderate wheeling without being "too much". Beyond that, if I choose to upgrade to 37's at some point, they will be "enough" to handle that size tire.
 

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Discussion Starter #4
^^^ n Thank you! I like that.... a simple straight forward way to get your answer:

Old gear x New Tire size / old tire size = New Gear

How cool is that?!
 

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^^^ n Thank you! I like that.... a simple straight forward way to get your answer:

Old gear x New Tire size / old tire size = New Gear

How cool is that?!
Actually its pretty great for the keep it simple lot (this guy). Gives me 4.74 and I'm going 4.88 thus making me feel like a near genius for making the choice I made! :worthy:
 

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^^^ n Thank you! I like that.... a simple straight forward way to get your answer:

Old gear x New Tire size / old tire size = New Gear

How cool is that?!
Yeah. But there is fine print. It's a good way to start the planning. You still have to consider other issues (tire weight and rolling resistance, intended use, etc.). But it is a straight forward way to get to the starting point for considering those other issues.
 

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Yeah. But there is fine print. It's a good way to start the planning. You still have to consider other issues (tire weight and rolling resistance, intended use, etc.). But it is a straight forward way to get to the starting point for considering those other issues.

Yeah, I get the fine print stuff.. and that's where it gets confusing, and to some degree it's not necessary for most of the folks who are asking in the first place. As stated, most of the folks looking for a guide are just trying to keep performance pretty close to the original. And the KISS rule is appropriate in this case.

When you start splitting hairs and really studying the fine print, that's when the intended audience is probably building something very specific. Over my head, and not what I'm in it for.

Thanks for all your help!
 

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Love the simple math. Plugged in my numbers and got 4.07, so 4.10's which is what I've been thinking all along.
 

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For those of us with 2012+ JK/JKU’s
There are so many threads asking the question: What gear ratio should I upgrade to?


I really like the Grimmjeeper's gear ratio calculator Gear Ratio Calculator


Basic guidelines for equating to stock. (*based on 255/75/17 stock tire size for 2012+ models) In this example I’m using 315/70/17 for y, as that seems to be the most popular tire size, and has a significant impact on real world driving, throttle response, and MPG.

Manual Transmissions – Stock equivalent
3.73’s stock = 4.10 upgrade (315/70/17) gives you <+100 RPM variance compared to stock in all gears.
4.10’s stock = 4.56 upgrade (315/70/17) gives you <+100 RPM variance compared to stock in all gears.
Automatic Transmissions – Stock equivalent
3.73’s stock = 4.10’s (315/70/17) gives you <+100 RPM variance compared to stock in all gears.
4.10’s stock = 4.56’s (315/70/17) gives you <+100 RPM variance compared to stock in all gears.

But here’s where it gets interesting! Using lower gear ratios that are not stock equivalent. Obviously, these are examples where you probably would not want to be if you’re mostly on road, but would be more useful if you’re building more to an off-road or crawler spec.
Manual 3.73 to 4.56 - The 3.73 – 5th gear is about the equivalent of 4.56 - 6th gear
Manual 4.10 to 5.13 - The 4.10 – 5th gear is about the equivalent of 5.13 - 6th gear

Auto 3.73 to 4.56 - The 3.73 – 4th gear is about the equivalent of 4.56 - 5th gear
Auto 4.10 to 4.88 - The 4.10 – 4th gear is about the equivalent of 4.88 - 5th gear

And finally, just an interesting little tidbit when you start cross-referencing manual transmission gear ratios to automatic transmission gear ratios, (tire size being equal
MT-3.73 vs. Auto-4.56 has almost identical RPM’s across the gears 1-5, but obviously you still have OD with the manny! This makes me feel better as I have the equivalent of an auto w/ 4.56's, but upgrading to 4.10's might just be nirvana.... and cheaper than getting a Rubicon!

So? Is this a good guideline or am I missing it?
Nice Work! Thanks for keeping it simple, just what I was looking for :thumb:
 

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Good points and i'm not an expert. As I have decided to re-regear I've recently gone through the same frustration and come to many similar conclusions. Nevertheless, when you add in different tire sizes, it gets even more interesting and when you factor in completely subjective things like driveability, or road feel, off road use to road use ratio..GAH!

That being said, my conclusion with my 2012 2 door rubicon manual transmission is that I should do 4.88 with 37s. If I had an auto, it would be 5.13. Completely subjective based on hours and hours of reading and I'm still not 100% that I'm correct!

Also, if you want cheap 4.10s when I'm done in 2-3 weeks, just PM!
Are you sure they are 4.10 and not 3.73? Reason I ask is that in 2012 and newer, the rubi's come stock with 3.73 and 4.10 is an option and not as common now.
 

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Initially I was split between 4.10's and 4.56. I think I would have been happy with 4.10's. They would have been "enough". But the 4.56 gears were a little easier to find (and maybe cheaper too, I forget). They're plenty for my daily driving and moderate wheeling without being "too much".
This was my EXACT thinking.. Orderded my 4.56's yesterday...:happyyes:
 

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I was chuckling when I read the first few paragraph of the original post because you are right it is confusing to many for three main reasons:

1) there are 2 different engine/transmission options (3.6 and 3.8) each of which has a difference performance curve
2) auto vs manual
3) there is no single correct answer in any given case (owner preferences)

The whole problem revolves (no pun intended) around how fast your engine turns (engine RPM) vs how fast you are going (wheel RPM) at cruising speed (60-70 MPH). Ideally you want the engine RPM to be near the 2400-3000 RPM range based on HP & torque curves. The relationship between the two RPM is optimized at the factory via gearing but when you change the tire size you change the relationship. You adjust to the change by changing the gears in the differential because it is much much cheaper than changing the gears in the transmission.

The calculator is useful but it doesn't tell the whole story like the Gear Charts, even though the charts were created with the same formula. The Gear Ratio charts were designed to be a simple tool but I don't think many understand them. It's important to understand them because changing gears is not cheap and it will make or break how you feel about driving your Jeep, so don't trust anyones advice blindly.

Pick the right chart for your engine (3.6 or 3.8) and transmission (auto or manual). Your current tire size and gear ratio can serve as reference point so you can see your current engine RPM at 70MPH (some charts use a different speed). You can also see the effect of using a slightly higher or lower gear ratio on engine rpm given your current tire size. The yellow band is the best choice for fuel economy (slightly less performance), the green band shows the best tradeoff between fuel economy and performance and the light blue shows slightly better performance (reduced fuel economy).

Pick the new tire size you are considering on the left column go across to the yellow, green or blue box depending on your bias towards mileage or responsiveness then go up to the top row and that gives you the gear ratio. That is literally all of the information you need. The problem is inexperienced owners ask other owners and there is a strong subjective element which is your preference for performance or fuel economy. What seems like a dog to one owner is Shangri-La to another and newbies get lost in the world of opinions.

The other subjective consideration is for manual transmission owners. A manual transmission can make up for incorrect gearing up to a certain point and with some consequences. For instance if the gears are too high ( low numerically such as 3.21s) then the driver can apply more gas to bring up the engine RPM and change the shift point accordingly. The problem is that you can use the loose the use of your top gear because the engine doesn't have enough power at cruising speed to maintain that gear.

I rarely see JK owners complaining that their gearing is too low (high numerically 5.13+); if that helps anyone. The challenge with tall gears (high numerically) is that the larger the ratio the larger the number of teeth on the gears which reduces their strength. This is particularly important to Sport and Sahara owners which have the Dana D30 differential in the front. Fortunately, the 3.6 engine eliminates the need for these gears with the popular 35" tires.

Use the gear chart and don't let others make a subjective decision for you!
 

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This may be the best gear thread yet. TONS of useful info and no, "you should do this", "man, you're going to hate that setup".

While I have already made my decison, some things said in here solidified it. This should be a great help(hopefully) to those just starting to search.

Thanks to all who have contributed
 

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First off, I have a 2014 JKU Sport Auto with 3.73's. I'm running 33" Duratracs. I do mostly highway driving back and forth to work. I have several hundred pounds of weight added to m rig also. I hate how I downshift every time I go up overpasses or drive in a strong head wind and would like to eliminate this. I guess what I am trying to say is that I want slightly better than stock performance. So if I am reading this correctly,

I have (3.73 X 33) / 32 = 3.84 rounded up should be 4:10?

But to gain performance instead of going back to stock ratio's I should go the next gear ratio up to 4:56's?????

I am torn between 4:10 and 4:56. As has been said before, when I go to 35's eventually I think 4:56 will be my better move as they would be "enough" and with my current 33's would put me in the "performance" range on the charts. Remember, I want better than stock performance so what's the opinion of you "Gear-heads"?
 

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What about the fact that most 35's are not really 35s when they sit on the car. My 315 70 17's are only 34" tall.
And boy, does my 2013 manual Moab suck on 3.73 with 35's. I would regear but this chart sends me to the rubicon 4.10 ratio. And rubi people complain about the same exact thing.
 
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