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Topic: Rpm's of a stock XJ at 60 mph.

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firehawk
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Rpm's of a stock XJ at 60 mph.
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What is the rpm a stock XJ with a 4.0, AW4, 3.55 gears and factory size tires at 60 mph running? 

Reason I'm asking is it seems by simply shifting out of overdrive the rpm's are back to what the Jeep ran when stock IN overdrive.  I am spinning at about 1600-1800 rpm IN overdrive with the 31" tires, and around 2200 rpm OUT of overdrive.  2200 rpm's seem to be at least at the lower end of the powerband, as it will tow the trailer and quads without any problems in "3rd".

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LEAD_NOT_FOLLOW
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You should not tow in OD on the AW4, says in both the AW4 Service Manual, FSM, and Factory User Manual. Put it in "3rd" if you are towing.

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firehawk
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I don't tow in overdrive. What I was wanting to know is what is the rpms in a stock XJ at 60 mph IN overdrive. As I said it seems that while in 3rd the rpms are back to what overdrive used to be before I mounted up the 31" tires.

What I do run into is while on the interstate in overdrive while not towing the Jeep will shift out of overdrive sometimes on even a slight hill. Funny thing is it will downshift sometimes on a certain hill and won't the next time I drive up it.

For instance when I had to drive to Princeton, KY for work on the way to Terre Haute that morning on I-70 it downshifted on several occasions. On the drive back that evening it didn't downshift once on the same hills.

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LEAD_NOT_FOLLOW
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According to the "math" aspect of it with the AW4.

AW4 - (1st) 2.804 | (2nd) 1.531 | (3rd) 1.000 | (OD) 0.753 | (Rev) 2.393

So with a factory sized tire (I used 28") this is what you'd be doing (based on 3.55 gears).

60 in 3rd = 2556rpm
60 in OD = 1924rpm

With a 31" sized tire this is what you'd be doing (based on 3.55 gears).

60 in 3rd = 2308rpm
60 in OD = 1738rpm

If you want to play around with this, I have it avaialble on our website here; http://www.greatlakesxj.com/tech/tech_calcs.html

Now like I said this is the MATH aspect of it, this does not take into effect wind resistance, extra weight, rolling resistance of a larger tire, etc.

As for why one time it shifted down and the other time it didn't shift at all. You have to factor in the time of day (heat does affect air in your tires), as well as the wind conditions. If your going into a head wind vs. having a tail wind that does make a big difference. Also if your on the same road but going the oposite direction nothing says that the grade (steepness) of the road is the same on both sides of the hill. Going the oposite direction means when you travel one way your going DOWN vs. going UP the other way. And lastly, it will matter what speed your going when you hit the hill as well. If you've got momentum (sp?) and say are going 60 and drop down to 50 while climbing vs say going 50 and trying to stay at 50 while climbing.

Point being, there are just way to many factors even on the same hill to give you a definate answer.

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xjfun
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trans down shift points are based on engine speed, wheel speed and engine vacuum. when engine speed is low, ie taller tires, and you approach a hill you give it more throttle vacuum drops, ie engine works harder. roads speed also will drop till you give it more throttle. ecu calabration says if x+y+z=a down shift to maintain road speed. the every other time shifting is very hard to diagnose. Hope this helps.

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firehawk
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xjfun wrote:

trans down shift points are based on engine speed, wheel speed and engine vacuum. when engine speed is low, ie taller tires, and you approach a hill you give it more throttle vacuum drops, ie engine works harder. roads speed also will drop till you give it more throttle. ecu calabration says if x+y+z=a down shift to maintain road speed. the every other time shifting is very hard to diagnose. Hope this helps.



I was always thinking along the same lines as Adam.  It has more to do with air temp and density in my case.  But I do see your point. 

Curious as to why the computer will cause it to downshift while in cruise, but if I have my foot on the gas I can get it up the hill without it downshifting.confused



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threeEs99
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Ok I tried leaving it in 3rd for a whole tank. Seemed to use more of 1st and 2nd RPM range also engine braked better.
I never expected it , but I got a gain of 20 more miles on my tank. WOO HOO :)
I'm giong to try it again, because I had 33s on it for a quarter of that tank. So we'll see.

-- Edited by threeEs99 at 18:35, 2008-08-08

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ChevelleSSLS6
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I've noticed on my motorcycle (2007 ninja 650r) that it has noticeably more torque when it's cool out than when it's warm- I can cruise up some hills at 35-40 in 6th gear (2500-3000rpm) when it's 55 degrees out, but when it's 75 I'll have do downshift it unless I want to lug the little engine and increase wear- which I obviously don't.

edit: I'm sure air temperature has some effect on power in our Jeeps as well.  I dont' notice it much with my xj, but perhaps the transmission controller does.

-- Edited by ChevelleSSLS6 at 09:51, 2008-08-11

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