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Old 12-30-2011, 08:28 AM   #1
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Gear Ratio Question

I'm curious about the advantages and disadvantages are to upgrading fron 3.73 gears to 4.10s.

Is it basically safe to say that I'll lose a bit in terms of mpg but will get more torque feel? How about highway driving? How will it impact my ability to do 75 on the highway?

I'm currently running 35" tires and think I should upgrade but I do alot of highway travel nd don't want to sacrifice the highway drivability.

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Old 12-30-2011, 08:38 AM   #2
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Automatic or manual? Going from 3.73 to 4.10 would be a waste. If you are going to change you should go lower.

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Old 12-30-2011, 08:47 AM   #3
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Manual. If I'm already at 2500 rpm at 75 mph should I even bother?
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Old 12-30-2011, 08:52 AM   #4
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3.6L or 3.8L?
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Old 12-30-2011, 09:21 AM   #5
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I'd not waste my time going from 3.73s to 4.10s.
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Old 12-30-2011, 09:27 AM   #6
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I have a 3.6. And why not waste time on 4.10? Seriously, break it down Elmo style for me!
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Old 12-30-2011, 09:28 AM   #7
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Quote:
Originally Posted by mflint1513
I have a 3.6. And why not waste time on 4.10? Seriously, break it down Elmo style for me!
Ok. That made me chuckle.
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Old 12-30-2011, 09:31 AM   #8
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Quote:
Originally Posted by mflint1513 View Post
I have a 3.6. And why not waste time on 4.10? Seriously, break it down Elmo style for me!
It wouldn't be cost effective. It would make more sense to go to at least 4.88 or 5.13.
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Old 12-30-2011, 10:25 AM   #9
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Numerically higher gears mean you'll be spinning higher RPMs at a given speed. I've attached a gearing chart that shows what happens at--I believe--65 mph.

If you look at a dyno chart, you'll see that torque is pretty even all the way across, but that hp keeps going up the higher the RPMs go.

Therefore, if you're presently at 2500 RPMs at 75 mph, and you regear so that you're at higher 3000 RPMs at 75 mph, you'll have more hp at the wheels after the regearing than you did before. The Jeep will feel more powerful. Of course, it also comes with all the downsides of running higher RPMs in terms of mpgs, engine wear and tear, and cabin noise.

What Daggo means about "cost effective" is that 3.73s in 6th gear with the 3.6 on 35" tires means that you're spinning about 2000 RPMs at 65 mph. If you went to 4.10s you'd be spinning about 2200 RPMs at the same speed. So you would've spent $1500 to regear for 300 RPMs. That would net you just a handful of horses, so that would be stupid.

You'd instead want to regear to say 4.88s, so you'd end up at something like 2600 RPMs. That would get you significantly more power, and might be worth $1500.
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Old 12-30-2011, 10:37 AM   #10
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I agree with everything MTH said-but has anyone looked at gears 2-5 for the ratios and rpms? I'm not thinking many of us regular for top gear passing or fuel Econ-it's for usable/passing numbers (lower gears).
Just a thought....
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Old 12-30-2011, 10:47 AM   #11
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Quote:
Originally Posted by rockjock1
I agree with everything MTH said-but has anyone looked at gears 2-5 for the ratios and rpms? I'm not thinking many of us regular for top gear passing or fuel Econ-it's for usable/passing numbers (lower gears).
Just a thought....
This is true. I for example (with the 3.8 on 3.21s and 33" tires in a 4 door) don't use 6th gear. I have no problem getting started from a stop, and so just dropping 6th gear on the highway basically makes it like I have 4.10s. Simple.
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Old 12-30-2011, 11:08 AM   #12
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Here's the thing: I don't have much issue on the highway even now. Some hills require me to downshift into 5th but in all, it seems ok. On the low side, taking off fro
a stop lacks that feel that my TJ had. I was thinking 4.10s would be a good compromise for all gears. So 4.88s seem to be the general concern us. What about 4.56s?
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Old 12-30-2011, 11:20 AM   #13
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Before you decide make sure you know what gears are available for your front axle. I know 5.13's are available for the d30 as I've decided on them when I get 35's. But I'm running the 3.8l with auto trans. Apples to oranges.
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Old 12-30-2011, 11:34 AM   #14
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Quote:
Originally Posted by mflint1513 View Post
Here's the thing: I don't have much issue on the highway even now. Some hills require me to downshift into 5th but in all, it seems ok. On the low side, taking off fro
a stop lacks that feel that my TJ had. I was thinking 4.10s would be a good compromise for all gears. So 4.88s seem to be the general concern us. What about 4.56s?
Until we get more folks doing regearings with their 2012s, it's going to be a little bit of guess work. Your 3.73s work. 4.10s would work. 4.56s would work. 4.88s would work. So long as you can get to 80 mph or so and still be a good way off from the redline, you should--technically--be fine. But what's the optimum power/mpg/comfort balance? I don't know, and it might vary some person-to-person.

New gears aren't really going to replicate low end torque--they'll just rev you more quickly so that the time between "no torque" and "torque" is less. The 4.0 had a little more low end torque (and a lot less weight to pull in a TJ) than the 3.8 in the JK, and the 3.6 has even less low end torque than the 3.8. That said, the 3.8 had a few more horses and the 3.6 has a lot more horses. The result is that the "take off from the stop" feeling with either the 3.8 or the 3.6 will not be as good as it was in your TJ, but passing somebody at 70 mph--especially with the 3.6--will feel a heck of a lot better. (The 3.8 JK also had a throttle by wire and auto tranny issues that are related but that's beside the point.)

A regearing will rev you a lot faster coming off a stop. You'll be shifting out of first very quickly. But, you'll also have more power at the wheels as a result during that same time.

I'd think 4.56s would be good, but it's really just a guess.
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Old 12-30-2011, 12:27 PM   #15
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Perfect ^^
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Old 12-30-2011, 01:00 PM   #16
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Quote:
Originally Posted by kjeeper10
Perfect ^^
X2

Thanks for all the input!
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Old 12-30-2011, 01:41 PM   #17
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I just got my 35s put on my 2012 Rubi with 3.73 gears.

It drives just great on the road. And, at least with the Rubi's 4.0/1.0 low range ratio, it should be more than low-enough off-road.

After a lot of reading and thinking, I was strongly considering going to 4.10 gears when I had the lift put on, but the lift installer suggested I drive it a bit before I spend $2K on something I might not need. I took his advice, and am glad I did.

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