Gears for 37" tires

[Oscar Indy]

I am certainly no gear expert but here is THE information you want...

Hey these are not for the JL this is JK info and completely wrong for the JT and JL.
Can you remove it please. The final drive ratios are off by over 400rpms for most gear sets

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[Oscar Indy]

So, looking at the above chart:

Stock 33" tires on a Rubicon with 4.10 gears = 2426 RPMs

And, running 37" tires at 2426 RPMs puts the gears in the 4.88 box

That chart is for the JK.

The JTR turns 1958 rpms in 8th at 70mph.
To match that on 37s you need 4.56 gears which will turn 1942 rpms at 70.
Or opt for a little deeper gears at 4.88 and turn 2078rpms at 70.
If you go 5.13 you will be turning the exact same rpms in 8th as you would be if you were using 4.10 gears and left it in 7th. I don't see the advantage of essentially removing the top gear.

I explained this in the video you dismissed btw.

 

[ShadowsPapa]

First Gear Ratio
4.71
Second Gear Ratio
3.14
Third Gear Ratio
2.11
Fourth Gear Ratio
1.67
Fifth Gear Ratio
1.28
Sixth Gear Ratio
1.00
Seventh Gear Ratio
0.84
Eighth Gear Ratio
0.67

Using these anyone can take the tire circumference and differential ratio and calculate RPM for a given highway speed.

 

[jurfie]

@Oscar Indy is right; that chart is for the JK and is not applicable to the JT. There is a thread on the JL Forum where people were talking about recreating it for the JL/JT with the new transmission, but no one had done it so far.

I’d do it (it is a simple excel spreadsheet with conditional formatting), but I haven’t had time to dig into the gear calculators.

 

[jrf]

And, running 37" tires at 2426 RPMs puts the gears in the 4.88 box

More like 4.56s is closer. I think this is a bit more area dependant also. If I was constantly climbing large mountains like in the rockies or such I think I'd be over gearing it a bit...if I lived in Iowa...I think under gearing it a bit.

For the little difference in RPMs we are talking about taking it off-road isn't going to matter much at all if even be noticeable with a 4:1 t-case.

I think the real answer is...it depends. 4.88s are probably the RIGHT overall gearing set considering going to 37s in addition to the diameter you are also adding mass. So to get the performance back 4.88s is your answer. Whether or not you need to do it is really up to what you do and where/how you drive. For me, it's not worth it. Most of the time when I'm not off-road or heading off-road I'm on flat roads and mostly under 45mph. When I do hit the highway they are VERY flat and I cruise at 75...I'll take the few RPMs less on the highway for the MPG gain. But that's my situation.

 

[jrf]

But, what's your highway speed?

70-75

 

[ShadowsPapa]

@Oscar Indy is right; that chart is for the JK and is not applicable to the JT. There is a thread on the JL Forum where people were talking about recreating it for the JL/JT with the new transmission, but no one had done it so far.

I’d do it (it is a simple excel spreadsheet with conditional formatting), but I haven’t had time to dig into the gear calculators.

Well gee I posted the JT automatic transmission gear ratios for ya! What's the problem LOL
J/K

If we were talking 1:1 as in 6th gear, it would be pretty simple but 8th is what, .67:1? Then you would plug in the various differential ratios, tire circumferences, and so on. Where's the math whiz here.

What's the top transmission gear ratio in the JK compared to the JT? Guess I could go look that up........

 

[Oscar Indy]

http://www.grimmjeeper.com/gears.html

The trans is ZF 8HP55
Use NVG241OR for Rubi trans case
Use NVG241 for sport / overland case.

Enter whatever you want to compare in the tire and gear settings.

Seriously charts are stupid. This works SOOOO MUCH BETTER.

 

[jurfie]

Well gee I posted the JT automatic transmission gear ratios for ya! What's the problem LOL
J/K

If we were talking 1:1 as in 6th gear, it would be pretty simple but 8th is what, .67:1? Then you would plug in the various differential ratios, tire circumferences, and so on. Where's the math whiz here.

What's the top transmission gear ratio in the JK compared to the JT? Guess I could go look that up........

Looks like this is it (FOR THE JK NOT THE JL/JT!!):

 

[jurfie]

http://www.grimmjeeper.com/gears.html

The trans is ZF 8HP55
Use NVG241OR for Rubi trans case
Use NVG241 for sport / overland case.

Enter whatever you want to compare in the tire and gear settings.

Seriously charts are stupid. This works SOOOO MUCH BETTER.

I've seen that site before, but it gives me a headache and is a bit overwhelming...I've got to wrap my head around it and read the instructions before tackling it. I'm really surprised no one smarter than me has done it yet, because I know a lot of people in the JK world rely on the charts as they are way more visual.

 

[ShadowsPapa]

I've seen that site before, but it gives me a headache and is a bit overwhelming...I've got to wrap my head around it and read the instructions before tackling it. I'm really surprised no one smarter than me has done it yet, because I know a lot of people in the JK world rely on the charts as they are way more visual.

And when you don't have reliable internet - you can download a chart and have it any time, any place, even in Alaska where there's no cell signals or internet access.
Here it's touch and go getting any internet so trying to mess with an online tool and plug in numbers and then woops - wait 2 minutes, ok it's back, start over.

Iowa - number 48 for broadband connectivity.

A chart works fine for comparison as you can see BOTH items being compared right next to each other, at the same time. OR even three options - what-ifs are piece of cake. Especially for visual people. But then people who are different are smacked here, so.......pretty obvious anyone who uses charts is stupid.

 

[Oscar Indy]

I've seen that site before, but it gives me a headache and is a bit overwhelming...I've got to wrap my head around it and read the instructions before tackling it. I'm really surprised no one smarter than me has done it yet, because I know a lot of people in the JK world rely on the charts as they are way more visual.

It's alot of info. Focus on the one thing you care about.
I love it because I can compare crawl ratios and fps in the low range.
A commonly missed part of regearing is what it will do to your crawl ratio. Too low isn't ideal

 

[jurfie]

Ok, I did a quick calculation using the Grimmjeeper calculator and the data @Oscar Indy provided.

FOR THE JT RUBICON w/8-SPEED AUTO; if you want 8th back on the highway at 70 MPH.

On the STOCK 285/70R17 tires:

4.10 gears = 1975 RPM​

35" tires (measuring 34"):

4.10 gears = 1900 RPM (-75)
4.56 gears = 2133 RPM (+158)
4.88 gears = 2262 RPM (+287)​

37" tires (measuring 36"):

4.10 gears = 1795 (-180)
4.56 gears = 1996 RPM (+21)
4.88 gears = 2136 RPM (+161)​

And as I play around with this, I'll again say @Oscar Indy is right; this calculator blows the charts out of the water because you can adjust for your exact conditions. I'll probably still put together a chart for the above conditions and include the other models gear ratio and tire sizes for convenience, but preface that it should only be used as a starting point.

 

[Oscar Indy]

Ok, I did a quick calculation using the Grimmjeeper calculator and the data @Oscar Indy provided.

FOR THE JT RUBICON w/8-SPEED AUTO; if you want 8th back on the highway at 70 MPH.

On the STOCK 285/70R17 tires:

4.10 gears = 1975 RPM​

35" tires (measuring 34"):

4.10 gears = 1900 RPM (-75)
4.56 gears = 2133 RPM (+158)
4.88 gears = 2262 RPM (+287)​

37" tires (measuring 36"):

4.10 gears = 1795 (-180)
4.56 gears = 1996 RPM (+21)
4.88 gears = 2136 RPM (+161)​

And as I play around with this, I'll again say @Oscar Indy is right; this calculator blows the charts out of the water because you can adjust for your exact conditions. I'll probably still put together a chart for the above conditions and include the other models gear ratio and tire sizes for convenience, but preface that it should only be used as a starting point.

First thing I did when looking at gears is I used this to compare the gear sets to my JK manual Rubi on 37s and 4.56 gears
Final drive was a concern but I really loved how my JK crawled. This calculator let me see down to the gear what I'd be looking at in both 4 low and high. This calculator also convinced me to avoid the manual in this truck. Comparing it to the JK the manual trans is very poorly geared to be running big tires.

 

[Oscar Indy]

And when you don't have reliable internet - you can download a chart and have it any time, any place, even in Alaska where there's no cell signals or internet access.

Thats a ridiculous scenario to justify a chart over a calculator. So here I am in the backcountry of nowhere making a final decision on what gears to go with............. have you even looked at the link? it'll run on dialup its just text and doesn't need connection to generate the math.

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