Gear ratio chart 2012 Auto - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 8 Old 10-10-2011, 06:45 AM Thread Starter
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Gear ratio chart 2012 Auto

Lookin for a chart for rpms, tire size, gear ratio.
2012 Auto
Thanks, Gene

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post #2 of 8 Old 10-10-2011, 12:00 PM
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The chart will be the same as it would be for any other vehicle. Just remember that they figure it out on a 1:1 ratio, which in a 3 speed would be "Drive". The 2012 JK's Overdrive id shown as .68 so you will have to figure it out accordingly, as the 4th gear is not really a 1:1 in this trans, but its a 1.4:1... so, out of overdrive its going to be a "little" higher RPM than the chart would show.

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post #3 of 8 Old 10-10-2011, 12:22 PM
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The 2012 5speed Auto (A580) gear ratios are:
1st - 3.59
2nd - 2.19
3rd - 1.41
4th - 1.0
5th - 0.83


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post #4 of 8 Old 10-10-2011, 12:25 PM
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Quote:
Originally Posted by Absolute Offroad View Post
The chart will be the same as it would be for any other vehicle. Just remember that they figure it out on a 1:1 ratio, which in a 3 speed would be "Drive". The 2012 JK's Overdrive id shown as .68 so you will have to figure it out accordingly, as the 4th gear is not really a 1:1 in this trans, but its a 1.4:1... so, out of overdrive its going to be a "little" higher RPM than the chart would show.
That information is different from what I have. Where did it come from?
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post #5 of 8 Old 10-10-2011, 12:52 PM
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Quote:
Originally Posted by Absolute Offroad View Post
The chart will be the same as it would be for any other vehicle. Just remember that they figure it out on a 1:1 ratio, which in a 3 speed would be "Drive". The 2012 JK's Overdrive id shown as .68 so you will have to figure it out accordingly, as the 4th gear is not really a 1:1 in this trans, but its a 1.4:1... so, out of overdrive its going to be a "little" higher RPM than the chart would show.
I believe the 3.8L chart probably took in the engine output at the given rpm to help determine what is best. 1500rpm for a V8 or 1500rpm for a 3.8L is a whole different selection of gear.

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post #6 of 8 Old 10-10-2011, 12:58 PM
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IMHO http://www.grimmjeeper.com/gears.html is the best gear ratio calculator out there. It allows for you to compare 2 different ratios side by side, gives road speed @ various rpms, etc.
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post #7 of 8 Old 10-10-2011, 01:04 PM
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For what it's worth, the final drive ratio of the new auto is very similar to the final of the old manual so you can use the chart that is on this site that is specific to the JK to figure your rpms.

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post #8 of 8 Old 10-11-2011, 06:03 AM
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Quote:
Originally Posted by Panamon Creel View Post
The 2012 5speed Auto (A580) gear ratios are:
1st - 3.59
2nd - 2.19
3rd - 1.41
4th - 1.0
5th - 0.83
Someone asked where the chart came from. It's from Alpars.

Here's the previous auto for comparison, the 42rle:

1st - 2.84
2nd - 1.57
3rd - 1.0
4th - 0.69

Reverse - 2.21

Huge improvement.

BTW, an easy enough way to get engine rpm's at a given speed with a specific set of tires and your transmission and axle ratio, with your transmission in any given gear is to multiply out the final drive ratio in that gear, then multiply that by the tire manufacturer's specified rotations per mile. This will give you engine rpm's at 60mph. Then divide 60mph into the speed for which you want to know rpm's. Then multiply that result by rpm's @ 60mph and you will have rpm's at that speed. Since there's some rounding and other factors like tire wear and inflation, it isn't exact but it is darned close.

An example:
2012 auto with 4.10's and 35" KM2's (model #38371. 35x12.5R17/D,) in 5th/OD on the highway at 70mph.
.83 (the 5th gear ratio) X 4.10 (axle ratio) = 3.403. Round to 3.40.
3.40 = final drive ratio.
BFG's spec'd revolutions/mile = 598
598 X 3.40 = 2033.2rpm @ 60mph. Round to 2033rpm @ 60mph.
70mph/60mph = 1.166666... Round to 1.17.
1.17 X 2033 = 2358rpm @ 70mph

At 50 mph:
50mph/60mph = .83333... Round to .833.
8.33 X 2033 = 1693rpm @ 50mph

JPK

Last edited by JPK; 10-11-2011 at 06:34 AM.
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