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Old 09-23-2012, 09:21 AM   #1
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High RPM's at highway speeds 2013 Rubicon

So I have noticed that my 2013 Rubicon unlimited 4 door with 4.10 gears is pushing 2600-2650 rpm's at 70 mph.

This seems high to me with the 5 speed transmission. My 2006 Rubicon with the 4 speed transmission has lower rpms at the same mph.

Tires are 265 for the 2006 and 285 for the 2013. Is that what is causing the higher rpms? I thought with the 5 speed transmission it would be lower.

On the 2013 I am getting about 16.5 mpg on the highway at 70 with the cruise on. This is just a little better than my 2006.

One other question is the 2013 supposed to have a skid plate under the motor? Mine does not.

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Old 09-23-2012, 09:44 AM   #2
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Does 06 have a4.10?

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Old 09-23-2012, 09:47 AM   #3
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Yes the 2006 Rubicon also has the 4.10 gears.
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Old 09-23-2012, 09:49 AM   #4
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Different engines. Both the 3.8 and your 3.6 operate most efficiently at higher rpms.
You can't compare it to the 4.0.
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Old 09-23-2012, 09:55 AM   #5
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I am not aware of an engine skid plate being standard.
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Old 09-23-2012, 09:58 AM   #6
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The 2006 has the 4.0 straight six.

Update just went outside to verify tire sizes. The tires are 245 for the 2006 and 255 for the 2013
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Old 09-23-2012, 10:03 AM   #7
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Quote:
Originally Posted by TNT4x4JK
The 2006 has the 4.0 straight six.

Update just went outside to verify tire sizes. The tires are 245 for the 2006 and 255 for the 2013
If you look at the charts for the auto 3.6 and 32" tires (actual 31") 4.10's do put you
you in the performance zone.

Isn't 4.10 an option over 3.73?

Put some 35" tires on there.... Problem fixed
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Old 09-23-2012, 10:06 AM   #8
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I don't know about the skid plate, but I can answer the rest of your question. I've got a 2013 Rubicon 2-dr with the 6-speed (0.797 top gear ratio + 4.10 axle), and stock tires (255/75R17, so they're 32" in diameter, or about 100" in circumference). The automatic's top gear ratio is 0.83, so pretty close. I hit 2000 RPM at/around 58 mph. I've got an hour long freeway commute to work (not for much longer, hopefully!), and cruising at 75 mph, it's around ~2600-2700 rpm (it's a little hard to tell exactly, especially since I can't maintain speed for long with the traffic). The gear ratios came directly from Chrysler, here: http://www.media.chrysler.com/dcxms/...nltd_Specs.pdf (Transmissions for the 2013's are the same.)

The math seems to support our observations - at 70 mph, the tires should be turning at 739 RPM. (70 mi/hr * 5280 ft/mi * 12 in/ft / (60 min/hr) / (100 in/rotation) = 739 rotations/min). With your 0.83 top gear and 4.10 axle, your final drive is 3.40. That times 739 RPM is 2513 RPM, the engine speed you should be seeing at 70 mph. There's been a lot of rounding in this process, so I think your number is reasonable. My assumption would be that the short axle is aimed at customers who want to upgrade tires without regearing.

FYI, I'm seeing right at 18 mpg during my 45-mile, 75 mph highway commute (with about 4 miles of stop and go traffic at ends).
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Old 09-23-2012, 10:07 AM   #9
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Oh, wait, if your tires are 285's, then my numbers are wrong. You should be seeing much lower RPM's than that...
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Old 09-23-2012, 10:11 AM   #10
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My 2013 sport is around 1800-2000 rpm's at 70 mph.
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Old 09-23-2012, 10:12 AM   #11
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Quote:
Originally Posted by Lyle
My 2013 sport is around 1800-2000 rpm's at 70 mph.
Gearing gearing my friend
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Old 09-23-2012, 10:12 AM   #12
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No your math is correct. I listed the wrong tire sizes in my first post. My 2013 has the stock 255 tires on it.

It just seemed like high rpm's to me. After looking at your post the rpm's seem to be normal.
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Old 09-23-2012, 10:36 AM   #13
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Quote:
Originally Posted by TNT4x4JK
No your math is correct. I listed the wrong tire sizes in my first post. My 2013 has the stock 255 tires on it.

It just seemed like high rpm's to me. After looking at your post the rpm's seem to be normal.
Ah, just saw your correction - so yeah, I think it's where it should be. The 3.6 is designed to rev higher than the 4.0. (Redline on my 2013 JKR is around 6400, while my '98 Cherokee was around 5000, IIRC.)
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Old 09-23-2012, 10:51 AM   #14
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The transmission gearing is different for 2012 and newer JKs, compared to pre-2012 JKs and TJs. The earlier transmissions had an overdrive ratio of something like 0.6:1. The new transmissions have an overdrive ratio of something like 0.8:1

My 2011 JK turned lower RPMs at highway speed with 5.38 gears in the axles than my 2012 JK does with 4.10 gears. The 4.10 gears border on being overkill with stock size tires. For someone that does a lot of highway driving 3.73 gears might be a better choice.

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