Yet Another Gearing Question

Frydaddy

Frydaddy

New member
2013 JKUS auto on Kanati 35" (34.5" actual) with 3.21:1?

I have not found a single cross reference to whats going on in my differentials.
- Build sheet from Jeep Site (linked) says 3.21:1
- Jacked up the rear and spun both tires once, drive shaft spins about 3 1/4 turns, sounds like 3.21:1
- at 70mph, and getting the torque converter to lock, it shows about 2000rpms, the below chart says that is 3.73:1
- I can almost guarantee cracking open the diff will show 3.21:1 considering the tire spin test.

The history is fairly straightforward. It was a rental jeep in Hawaii for two years. Then bought at auction and a dealer slapped on some tires and pucks under the springs. I have the Superchips Flashpaq and updated the tire size, the speedometer matches GPS tests.

This concern all comes about because I still don't like the way it shifts (shoulda' got a stick, I know). I do a lot of highway driving 65-80mph, and its almost constantly shifting. The general consensus is that 4.10 would restore the stock final ratio, and 4.56 gives a good ratio for highway mileage and crawling.

Looking at the chart below. I have two situations that might happen.
- If i fudge the tire size to match RPM with Ratio (30"), 4.56 will put you a little high at 2968rpm, and 4.10 gives a nice 2669rpm
- If I fudge the ratio to match Tire Size with RPM (3.73), 4.56 will be perfect at 2544rpm, but 4.10 is a laughably small change

3.6-jkgearing.jpg


TLDR: All evidence shows 3.21, EXCEPT real highway results suggest 3.73.
Thoughts?

Edit: Link broken. Follow the link, then click on "Equipment Listing"
 
Last edited:
Frydaddy said:
2013 JKUS auto on Kanati 35" (34.5" actual) with 3.21:1?

I have not found a single cross reference to whats going on in my differentials.
- Build sheet from Jeep Site (linked) says 3.21:1
- Jacked up the rear and spun both tires once, drive shaft spins about 3 1/4 turns, sounds like 3.21:1
- at 70mph, and getting the torque converter to lock, it shows about 2000rpms, the below chart says that is 3.73:1
- I can almost guarantee cracking open the diff will show 3.21:1 considering the tire spin test.

The history is fairly straightforward. It was a rental jeep in Hawaii for two years. Then bought at auction and a dealer slapped on some tires and pucks under the springs. I have the Superchips Flashpaq and updated the tire size, the speedometer matches GPS tests.

This concern all comes about because I still don't like the way it shifts (shoulda' got a stick, I know). I do a lot of highway driving 65-80mph, and its almost constantly shifting. The general consensus is that 4.10 would restore the stock final ratio, and 4.56 gives a good ratio for highway mileage and crawling.

Looking at the chart below. I have two situations that might happen.
- If i fudge the tire size to match RPM with Ratio (30"), 4.56 will put you a little high at 2968rpm, and 4.10 gives a nice 2669rpm
- If I fudge the ratio to match Tire Size with RPM (3.73), 4.56 will be perfect at 2544rpm, but 4.10 is a laughably small change

View attachment 233792


TLDR: All evidence shows 3.21, EXCEPT real highway results suggest 3.73.
Thoughts?

Edit: Link broken. Follow the link, then click on "Equipment Listing"
Click to expand...

The reason your highway test results the way it does, is because you're actually in fourth gear.
Use your manual shifter next time your at highway speeds. You aren't reaching 5th with 3:21's.
These charts are based on a reading in your highest gear, in this case fifth.

**Also, I wouldn't put misinformation in the ECU to try and "fudge" the RPM's.
Your transmission is designed to shift based upon speed, if you change this to something inaccurate, it will drive like poo.
It's not going to shift correctly again until you re-gear, even with the speedometer corrected.
You have too much load on the transmission to properly hold a gear. Basically it acts as if you are towing something.
 
Last edited by a moderator:
JKDream said:
The reason your highway test results the way it does, is because you're actually in fourth gear.
Use your manual shifter next time your at highway speeds. You aren't reaching 5th with 3:21's.
These charts are based on a reading in your highest gear, in this case fifth.

**Also, I wouldn't put misinformation in the ECU to try and "fudge" the RPM's.
Your transmission is designed to shift based upon speed, if you change this to something inaccurate, it will drive like poo.
It's not going to shift correctly again until you re-gear, even with the speedometer corrected.
You have too much load on the transmission to properly hold a gear. Basically it acts as if you are towing something.
Click to expand...


Yes, I used the shifter to get in fifth.

Sorry if I wasn't clear, The "fudging" was done ONLY in reference to the chart. I had three disparate data points but only need two to use this chart. The logical thing is to compare the two combinations. The bullets, were my results. The ECU stayed at 3.21:1 and 34.5" the whole time.

I understand the need to re-gear, thats why I asked. Until then, Secretly, I hoped the corrected speedometer would give the TCU an accurate reference for speed vs RPM, and therefore shift pattern, but no, that would make too much sense for FCA (/scarcasm). Question still stands, why does it act like 3.73 on the road, when all the paperwork says 3.21?
 
Frydaddy said:
Yes, I used the shifter to get in fifth.

Sorry if I wasn't clear, The "fudging" was done ONLY in reference to the chart. I had three disparate data points but only need two to use this chart. The logical thing is to compare the two combinations. The bullets, were my results. The ECU stayed at 3.21:1 and 34.5" the whole time.

I understand the need to re-gear, thats why I asked. Until then, Secretly, I hoped the corrected speedometer would give the TCU an accurate reference for speed vs RPM, and therefore shift pattern, but no, that would make too much sense for FCA (/scarcasm). Question still stands, why does it act like 3.73 on the road, when all the paperwork says 3.21?
Click to expand...

The chart isn't entirely accurate as well.
There's no answer regarding tire size, if the 37 listed is a marked 37 or a true 37 height.
I can tell you that my RPM with 37 MTR's and 5:13s is not what is listed on the sheet.
The manual shifters on these are more of an assist rather than a stick style shifter.
Even though you may have put it into fifth, the torque converter is likely keeping you in fourth.
An example, try putting it into second or third while completely stopped, then accelerate.
You'll start as if you are in first gear, because you actually are.
For reference, I was on 3:21's auto and 35's as well prior.

** Not trying to give you a hard time. Personally I just don't believe the chart to be overly accurate.
It might be within a range, but my results on 3:21s and 5:13s was about ~200-300rpms off.
The way the transmission shifts even re-geared is odd, that's why I don't think you actually got the torque converter to lock in fifth.
 
Last edited by a moderator:
I don't know why you are having weird results. With your size tires, I'd go at least 4.56, and personally, I'd prefer 4.88. From everything I've read, it won't make a terrible difference in gas mileage, and the lower range is much more desirable, off road.


Sent from my iPhone using WAYALIFE mobile app
 
JKDream said:
The chart isn't entirely accurate as well.
There's no answer regarding tire size, if the 37 listed is a marked 37 or a true 37 height.
I can tell you that my RPM with 37 MTR's and 5:13s is not what is listed on the sheet.
The manual shifters on these are more of an assist rather than a stick style shifter.
Even though you may have put it into fifth, the torque converter is likely keeping you in fourth.
An example, try putting it into second or third while completely stopped, then accelerate.
You'll start as if you are in first gear, because you actually are.
For reference, I was on 3:21's auto and 35's as well prior.

** Not trying to give you a hard time. Personally I just don't believe the chart to be overly accurate.
It might be within a range, but my results on 3:21s and 5:13s was about ~200-300rpms off.
The way the transmission shifts even re-geared is odd, that's why I don't think you actually got the torque converter to lock in fifth.
Click to expand...

Yep, sounds about right. The chart is probably off.

I read the side shifing on the autos only limit the top gear, so it wont change any patterns below the selected gear. My testing involved getting up to 80, then throttling back until i saw the needle drop into another gear. I got good at throttle-dancing with the tranny in my old XJ.

Do the JK trannys have torque locks? My old XJ would upshift late then lock the torque converter early, making any slight hill turn into either lugging the engine, or making it scream! (completely skipping top gear, unlocked.) I ended up splicing a three way switch into the electric clutch. the positions were: Stock (TCU control), Unlocked, and Locked. It was great having the extra control.
 
notnalc68 said:
I don't know why you are having weird results. With your size tires, I'd go at least 4.56, and personally, I'd prefer 4.88. From everything I've read, it won't make a terrible difference in gas mileage, and the lower range is much more desirable, off road.
Sent from my iPhone using WAYALIFE mobile app
Click to expand...

You're right, I think 4.56 would be the way to go.

Assuming the chart is close enough, I think 2500rpm at 70 would be good, considering i'm on the highway all the time.
But if the chart is wrong, 4.56 could give me 2900rpm at 70. That sounds a bit high, the engine would be screaming everywhere I went.
 
Frydaddy said:
You're right, I think 4.56 would be the way to go.

Assuming the chart is close enough, I think 2500rpm at 70 would be good, considering i'm on the highway all the time.
But if the chart is wrong, 4.56 could give me 2900rpm at 70. That sounds a bit high, the engine would be screaming everywhere I went.
Click to expand...

IDK what it is. My Rubicon with auto tranny, is very close to the chart, when using actual measured tire size.

My tires measured 33.5, and I'm running just below 2500 at 70, with 4.10 gears.


Sent from my iPhone using WAYALIFE mobile app
 
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notnalc68 said:
IDK what it is. My Rubicon with auto tranny, is very close to the chart, when using actual measured tire size.
My tires measured 33.5, and I'm running just below 2500 at 70, with 4.10 gears.
Sent from my iPhone using WAYALIFE mobile app
Click to expand...

Wow, i guess i'm nit-picking over a 200rpm difference. It makes a big difference later, but that is within the margin of error, and there's so much evidence to the contrary. I should just shut-up and go for it.

So, gears, carriers, maybe limited slip rear (save the front to lock later)... I've done my own gears before, but I doubt I have the time anymore. Shop quotes after the holiday debt.
 
Frydaddy said:
Wow, i guess i'm nit-picking over a 200rpm difference. It makes a big difference later, but that is within the margin of error, and there's so much evidence to the contrary. I should just shut-up and go for it.

So, gears, carriers, maybe limited slip rear (save the front to lock later)... I've done my own gears before, but I doubt I have the time anymore. Shop quotes after the holiday debt.
Click to expand...

Check out this article...

http://project-jk.com/jeep-jk-tech/do-yourself-a-favor-and-regear-your-2012-up-jeep-jk-wrangler


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