New Pentastar Engine - Not impressed so far
12-31-2011, 06:57 AM
#1
JK Enthusiast
Thread Starter
New Pentastar Engine - Not impressed so far
In regards to the gas mileage.
I have a new 2012 JK Unlimited Rubicon with the 3.6 pentastar engine and new 5 speed transmission. I just came from a Unlimited Rubicon with the 4 speed and 3.8 "mini van" motor. The extra power in the 3.6 is much appreciated and noticeable right off the bat, however, something doesn't seem right. 1,900 RPMs to cruise at 56 MPH? The old JK could do the same at 1,600 RPMs and get over 24 MPG highway. I don't understand why a newer more powerful engine would take more RPMs to cruise at the same speed?
I'm getting around, I don't know, 20-21 MPGs highway at 56 with the new engine and worse figures with higher cruise control speeds. Granted there are only 120 some odd miles on it so far, but the old 3.8 would get better gas than that right off the bat, and never took 1,900 RPMs to cruise at that speed.
Anyone able to shed some light on the situation here?
I have a new 2012 JK Unlimited Rubicon with the 3.6 pentastar engine and new 5 speed transmission. I just came from a Unlimited Rubicon with the 4 speed and 3.8 "mini van" motor. The extra power in the 3.6 is much appreciated and noticeable right off the bat, however, something doesn't seem right. 1,900 RPMs to cruise at 56 MPH? The old JK could do the same at 1,600 RPMs and get over 24 MPG highway. I don't understand why a newer more powerful engine would take more RPMs to cruise at the same speed?
I'm getting around, I don't know, 20-21 MPGs highway at 56 with the new engine and worse figures with higher cruise control speeds. Granted there are only 120 some odd miles on it so far, but the old 3.8 would get better gas than that right off the bat, and never took 1,900 RPMs to cruise at that speed.
Anyone able to shed some light on the situation here?
12-31-2011, 07:35 AM
#3
JK Freak
Its because the new 5 speed auto OD ratio is 0.83 and the previous 4 speed auto OD ratio is 0.69. That's why 3:73s would be better for highway mpg. Also that's why we felt like we were pushing a rope in OD with the old auto. But the plus is better crawling with 3:59 first gear ratio compared to 2.84 in the previous autos, it's also lower in 2nd and 3rd gear.
One of the first posts I read about the 2012s said that in 1st gear, the 2012 with 3:73s is a better crawler than the earlier transmissions with 4:10s and the 2012 with 4:10s is a slightly better crawler than the earlier transmissions with 5:33s.
The down side is the higher OD ratio.
One of the first posts I read about the 2012s said that in 1st gear, the 2012 with 3:73s is a better crawler than the earlier transmissions with 4:10s and the 2012 with 4:10s is a slightly better crawler than the earlier transmissions with 5:33s.
The down side is the higher OD ratio.
12-31-2011, 07:41 AM
#4
JK Super Freak
my old 08 with 3.21 would not even run 1600rpm at 56mph, 2000rpm maybe, my 2010 rubi 6 speed runs 2200 rpm at about 56mph. I don't understand what your complaining about, first its a jeep, 2nd a rubi with 4.10's, 3rd its an automatic, and finially of the mall crawling persuasion. 
I think your doing pretty good. just my 
I think your doing pretty good. just my
12-31-2011, 07:58 AM
#5
JK Enthusiast
Thread Starter
The old rubicons always came with 4.10s, and far as I know, the 2012s are the same. However, my old set up had the 4.10s, and got great gas mileage.
It matters because I love having a jeep, however, I can only afford to have one vehicle and thus it is my daily driver, and I commute a lot. With the old rigs, I was able to have the best of all worlds by still getting good gas mileage.
Any suggestions that won't break the bank? IE, paying $2k to set up a lift and 35s isn't worth it, nor is downgrading to an X (I'd rather go back to my old JK Rubi).
It matters because I love having a jeep, however, I can only afford to have one vehicle and thus it is my daily driver, and I commute a lot. With the old rigs, I was able to have the best of all worlds by still getting good gas mileage.
Any suggestions that won't break the bank? IE, paying $2k to set up a lift and 35s isn't worth it, nor is downgrading to an X (I'd rather go back to my old JK Rubi).
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12-31-2011, 12:11 PM
#8
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I have 2012 with 6sp manual and 4:10s. Once it was broke in I was impressed. I have heard rumors that the computer detunes during break in. I drove a 2008 for a while and it is night and day.
12-31-2011, 01:22 PM
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The old rubicons always came with 4.10s, and far as I know, the 2012s are the same. However, my old set up had the 4.10s, and got great gas mileage.
It matters because I love having a jeep, however, I can only afford to have one vehicle and thus it is my daily driver, and I commute a lot. With the old rigs, I was able to have the best of all worlds by still getting good gas mileage.
Any suggestions that won't break the bank? IE, paying $2k to set up a lift and 35s isn't worth it, nor is downgrading to an X (I'd rather go back to my old JK Rubi).
It matters because I love having a jeep, however, I can only afford to have one vehicle and thus it is my daily driver, and I commute a lot. With the old rigs, I was able to have the best of all worlds by still getting good gas mileage.
Any suggestions that won't break the bank? IE, paying $2k to set up a lift and 35s isn't worth it, nor is downgrading to an X (I'd rather go back to my old JK Rubi).
