Gear Ratio Tire Size Chart - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 13 Old 02-22-2011, 07:51 AM Thread Starter
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Gear Ratio Tire Size Chart

This is a replacement to fix a broken link in the library. Thanks to Planman for this post I found with a lot of good info and explanations of how and why.

Quote:
Originally Posted by planman View Post
This is why we need lower gears than the 4.0L straight six engine models did:

(Stock figures in the left column. For example, notice the 64 rear wheel HP at 2k rpm.)



Then, you should consider that the 4th gear (overdrive) for an automatic is a very tall 0.69, and the 6th gear ratio for a stick is 0.84.

So then, accounting for our engine horsepower/rpm information, the additional HP (thus, higher rpm) needed over stock to turn larger tires, you end up with this:




Then, consider all the abuse/hard wheeling of the guys on these and other forums with 5.13s and 5.38s. I don't ever recall a rig with 37s or smaller busting a gear ring tooth without having it being caused first by the load/unload of a broken shaft or u-joint as the weaker link. In other words, experience on these forums seems to suggest that even with 5.13s or 5.38s installed in the JK Dana 44s, if they are properly installed, they are not the weakest link in the system.

So, the concerns that the smaller teeth of 5.38s on a JK Dana 44 would be too weak appear to be unfounded.

Again, some of this is because so many of us are used to the smaller ring size on the TJ Dana 44 axles. The JK Dana 44 axles have a significantly larger ring diameter. So, 5.13s and 5.38s have teeth that are not as small as a 5.13 on a TJ Dana 44.

Because I live and wheel at a higher elevation here in Montana, an automatic with 35s or larger should go with 5.38s up here. We need the higher rpm levels to have enough rear wheel HP to drive the highway/freeway grades with lower oxygen levels. For a stick, 4.88s with 35s and 5.13s with 37s on a 2 door, and 5.13s for 35s or 37s for a 4 door.

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post #2 of 13 Old 08-23-2012, 09:26 AM
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I see that this chart is built to 70mph. Is there any type of chart for a lower range? I typically don't go over 60mph with my Jeep on the highway. But, that may be because I haven't regeared yet.
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post #3 of 13 Old 05-02-2013, 11:57 AM
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Since I have been looking at gear choices I wanted to add to this thread. The chart above is based on 70 mph. Seeing as not all of us cruise at 70 mph I wanted to add in the math so anyone could figure out what they could achieve at lower speeds. I typically cruise around 60 mph and I feel that I get the best fuel economy when my engine rpm's are around 2100 so i will be looking for a gear set that puts me in that range at 60 mph.

A couple key things to know are:

-There are 63360 inches in 1 mile.
-Knowing the actual diameter of your tires will give the best results. For the sake of this post I will be using my tire size. A Nitto Terra Grappler in 295x70r18 with an actual diameter of 34.41"
-The circumference of a circle= Diameter x Pi (it's a button on your calculator).


So we begin by finding the circumference of our tire.. 34.41 x Pi = 108.1
This says my tire will travel 108.1 inches in 1 revolution. I know that there are 63360 inches in a mile so by dividing that number by 108.1 tells me that my tire will rotate 586.12 times in 1 mile. At 60 mph that also equates to 586.12 rpm. (if you want to work at any other speed you need to multiply the number of tire rotations in a mile by the speed you drive. Then divide that number by 60 and you will have the number of rpm's for your tire).

Next is looking at the gear ratio for your ring and pinion. Remember, the ratio is pinion to ring gear. If i choose a 5.13 ratio that means the pinion is turning 5.13 times for every 1 time your ring gear turns. (or you can think of it as your driveshaft turns 5.13 times for every rotation of your tire). So the math would look like this: tire rpm x gear ratio = driveshaft rpm
586.12 x 5.13 = 3006.8

We know that 4th gear in my 08 auto trans is .69
So we take 3006.8 and multiply that by .69.
3006.8 x .69 = 2074.68

So with my tires, a 5.13 gear set and me driving 60 mph. My engine rpm's will be about 2075. Thats not bad.

If I step up to 5.38's the last bit of math looks like this:
586.12 x 5.38 = 3153.32

3153.32 x .69 = 2175.79
At 60 mph I would see engine rpm's at 2176.

If you have the manual trans you would change the .69 to your last (6th) gear ratio.

I will put all of this into a simple to use excel sheet. If anyone wants it I will email it to you. With the sheet you would just have to input your tire diameter, cruzing speed and axle gear ratio. It will do the math and tell you what your engine RPM's will be.

Hope this helps!

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post #4 of 13 Old 05-06-2013, 06:47 AM
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Quote:
Originally Posted by AWOL View Post
Since I have been looking at gear choices I wanted to add to this thread. The chart above is based on 70 mph. Seeing as not all of us cruise at 70 mph I wanted to add in the math so anyone could figure out what they could achieve at lower speeds. I typically cruise around 60 mph and I feel that I get the best fuel economy when my engine rpm's are around 2100 so i will be looking for a gear set that puts me in that range at 60 mph.

A couple key things to know are:

-There are 63360 inches in 1 mile.
-Knowing the actual diameter of your tires will give the best results. For the sake of this post I will be using my tire size. A Nitto Terra Grappler in 295x70r18 with an actual diameter of 34.41"
-The circumference of a circle= Diameter x Pi (it's a button on your calculator).


So we begin by finding the circumference of our tire.. 34.41 x Pi = 108.1
This says my tire will travel 108.1 inches in 1 revolution. I know that there are 63360 inches in a mile so by dividing that number by 108.1 tells me that my tire will rotate 586.12 times in 1 mile. At 60 mph that also equates to 586.12 rpm. (if you want to work at any other speed you need to multiply the number of tire rotations in a mile by the speed you drive. Then divide that number by 60 and you will have the number of rpm's for your tire).

Next is looking at the gear ratio for your ring and pinion. Remember, the ratio is pinion to ring gear. If i choose a 5.13 ratio that means the pinion is turning 5.13 times for every 1 time your ring gear turns. (or you can think of it as your driveshaft turns 5.13 times for every rotation of your tire). So the math would look like this: tire rpm x gear ratio = driveshaft rpm
586.12 x 5.13 = 3006.8

We know that 4th gear in my 08 auto trans is .69
So we take 3006.8 and multiply that by .69.
3006.8 x .69 = 2074.68

So with my tires, a 5.13 gear set and me driving 60 mph. My engine rpm's will be about 2075. Thats not bad.

If I step up to 5.38's the last bit of math looks like this:
586.12 x 5.38 = 3153.32

3153.32 x .69 = 2175.79
At 60 mph I would see engine rpm's at 2176.

If you have the manual trans you would change the .69 to your last (6th) gear ratio.

I will put all of this into a simple to use excel sheet. If anyone wants it I will email it to you. With the sheet you would just have to input your tire diameter, cruzing speed and axle gear ratio. It will do the math and tell you what your engine RPM's will be.

Hope this helps!
AWWWWW To many maths!!!!!

Thanks, I was considering regearing to 4.88, but it seems like that may not be enough.

What is your tipping point?
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post #5 of 13 Old 05-06-2013, 10:12 AM
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It was lengthy yes. That was about as short as I could type it. I could email you the excel sheet I made. LOL it does the math for you.

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post #6 of 13 Old 05-28-2013, 06:08 AM
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Could I get a copy of your spreadsheet?

Froggie
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post #7 of 13 Old 05-29-2013, 05:16 AM
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Quote:
Originally Posted by Froglips View Post
Could I get a copy of your spreadsheet?

Froggie
I can email it to you. PM me your email address.


Eric.

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post #8 of 13 Old 05-29-2013, 07:15 AM
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There's a really great online calculator that does all the math for you so you can make your own chart at whatever speed you want. You don't even have to look up the gear ratios for the transmission, just pick it from a drop down menu.

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

I hear the guy who made it has some pretty cool Jeeps.


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Last edited by grimmjeeper; 05-29-2013 at 08:06 AM.
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post #9 of 13 Old 08-22-2013, 06:29 AM
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is there a chart for the 2012+ 5 speed autos? and the 3.6L ?

or Grimm, what trans should I select for the 2012 5 speed auto in your calculator?
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post #10 of 13 Old 08-22-2013, 07:34 AM
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http://www.4ws.com/gear-ratio-rpm-calculator

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post #11 of 13 Old 09-09-2013, 07:06 PM
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Quote:
Originally Posted by swampjeep View Post
is there a chart for the 2012+ 5 speed autos? and the 3.6L ?

or Grimm, what trans should I select for the 2012 5 speed auto in your calculator?
Anyone 3.6L chart?

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post #12 of 13 Old 04-17-2015, 05:00 AM
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How about a gear chart with an 545RFE & 5.7 HEMI?



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post #13 of 13 Old 04-17-2015, 06:00 AM
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Quote:
Originally Posted by Got_Gun? View Post
Anyone 3.6L chart?
Don't need a chart specific to the 3.6 or any other engine. You need to know the gear ratios of your transmission and differential.

This is my favorite site for such calculations, he also has other calculators.

http://www.4lo.com/calc/gearratio.php - With this calculator, you plug in all but one piece of information and it will then calculate that entry.
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