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Old 05-04-2011, 10:59 PM   #1
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70 mph rpm?

Model: 2011 JK Rubicon
Tires: 32 inch AT
transmission: 6 speed
Diff: 4.10
RPM @ 70 MPH = 2400

Curious as to what RPM's other people are getting with different combinations?

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Old 05-04-2011, 11:09 PM   #2
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My stock Rubicon tachs 2400 RPM at 65 MPH in 6th gear.

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Old 05-05-2011, 08:53 AM   #3
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'08 JKU Rubi-

Interpolating the speedo, looks like about 2700rpm at 70 mph-O/D

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Old 05-05-2011, 09:04 AM   #4
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Model: 2008 JKU Sahara
Tires: 35 inch MT
Transmission: 6 speed
Diff: 4.88
RPM @ 70 MPH = 2900
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Old 05-05-2011, 09:08 AM   #5
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Quote:
Originally Posted by Mikial View Post
Curious as to what RPM's other people are getting with different combinations?
Take a look at the chart in this post. It lays out the RPMs a JK will run with various tires sizes and gearings. Note that there's actually two charts, one for an auto and the other for a manual.
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Old 05-05-2011, 09:21 AM   #6
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Quote:
Originally Posted by MTH

Take a look at the chart in this post. It lays out the RPMs a JK will run with various tires sizes and gearings. Note that there's actually two charts, one for an auto and the other for a manual.
That just gives you an idea of where your RPMs will be. Real life numbers can be different.
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Old 05-05-2011, 09:39 AM   #7
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Well the chart is exact using a mathematical formula based on transmission ratio to gear ratio, tires size and speed. If you use the correct numbers then the output has to be exact match to the chart.

Where the varible comes into play is the fact that an example 33" tire is not exactly 33"'s. Depending on inflation of your tires a 33" is more likely around 32" due to compression of the tire to the road from the weight of the vehicle. Thats why when you use programmers to change tire size you never use the actually tire size. If you put on 33" and set your computer to 33" tire your speedo will still be off.

For the most part you can subtract close to 1" from what your tire is according to the manufacture and you will come close to actual size on your Jeep.
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Old 05-05-2011, 09:48 AM   #8
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Well thats true, but


Quote:
Originally Posted by rics1997 View Post
Well the chart is exact using a mathematical formula based on transmission ratio to gear ratio, tires size and speed. If you use the correct numbers then the output has to be exact match to the chart.

Where the varible comes into play is the fact that an example 33" tire is not exactly 33"'s. Depending on inflation of your tires a 33" is more likely around 32" due to compression of the tire to the road from the weight of the vehicle. Thats why when you use programmers to change tire size you never use the actually tire size. If you put on 33" and set your computer to 33" tire your speedo will still be off.

For the most part you can subtract close to 1" from what your tire is according to the manufacture and you will come close to actual size on your Jeep.
My tires are 33.2" installed, however at 70mph-centrifugal force increases the tire diameter to-probably 33.3/33.4 and this inputs a fractional error in the speedo, need a study for that fig. !!

I ain't got the lab/instr. forit--

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Old 05-05-2011, 10:08 AM   #9
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Quote:
Originally Posted by ZOMB1E View Post
Model: 2008 JKU Sahara
Tires: 35 inch MT
Transmission: 6 speed
Diff: 4.88
RPM @ 70 MPH = 2900
Quote:
Originally Posted by ZOMB1E View Post
That just gives you an idea of where your RPMs will be. Real life numbers can be different.
Quote:
Originally Posted by rics1997 View Post
Where the varible comes into play is the fact that an example 33" tire is not exactly 33"'s. . . . . For the most part you can subtract close to 1" from what your tire is according to the manufacture and you will come close to actual size on your Jeep.
Right. And sure enough, Zombie's 35s are really turning the RPMs for about 34s, which according to the chart would be around 2837. It's going to be pretty darn close to "real life." The charts that are off are the ones that are more generic and don't anticipate the proper overdrive gearings for the JK.

By the way Zombie, do you spend much time on the highway? Do you feel like you're overrevving? I'm considering 4.56s for 33s which would similarly put me around ~2800 RPM at 70 mph.
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Old 05-05-2011, 11:45 AM   #10
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Quote:
Originally Posted by MTH

Right. And sure enough, Zombie's 35s are really turning the RPMs for about 34s, which according to the chart would be around 2837. It's going to be pretty darn close to "real life." The charts that are off are the ones that are more generic and don't anticipate the proper overdrive gearings for the JK.

By the way Zombie, do you spend much time on the highway? Do you feel like you're overrevving? I'm considering 4.56s for 33s which would similarly put me around ~2800 RPM at 70 mph.
The 3.8 seems to have a nice steady hum around 2800 RPM and you can definitely tell you are right in the powerband. I generally look at the tach first and the speedo second when im driving.

Going from a V8 sports car to a JK, I often feel like i'm going to blow the engine when I rev it much over 3k. I was so used to always hearing the sound of the exhaust and not just the engine. I'm pretty sure the extra RPMs are not causing excessive wear, but it's something I still haven't gotten used to even after 10k miles on the new gears.

Not really sure if I answered your question. The highway I have no problems with. The only thing I don't care for is how short 1st gear is when accelerating from a dead stop.
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Old 05-05-2011, 11:57 AM   #11
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^^Answered my question perfectly, thanks. I'm just a little skiddish about having no choice (absent coasting in neutral) but to be up around 3k RPMs on the highway. Sounds like it's working out pretty good for you though . . . I think I'm slowly coming around.

I've heard 1st gets really short. Apparently some people decide to start in 2nd and skip 1st altogether.
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Old 05-05-2011, 12:09 PM   #12
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This 3.8L is really comfy 2500/3500 rpm and my milege is steadily increasing as I break the gears in-

Mileage started at 15 mpg (computer) and now after 60 miles (3-20 mile runs) it's showing 15.6 mpg--

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Old 05-05-2011, 12:34 PM   #13
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Quote:
Originally Posted by JIMBOX View Post
This 3.8L is really comfy 2500/3500 rpm and my milege is steadily increasing as I break the gears in-
That's interesting. I will say that with a manual, shifting up at 3500+ has a different feel than shifting down lower.

Once I depress the clutch at 3500+, I feel like I want to wait a moment for the revs to come back down before letting off the clutch in a higher gear. Something about the sound and feel of the engine spinning that fast without the load of the transmission makes releasing the clutch before the revs come down feel somehow . . . alarming. Especially up toward 4000 RPM. Maybe Zombie knows what I'm talking about.

Perhaps the auto overcomes this . . . or perhaps I am just making up a problem that doesn't exist.
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Old 05-05-2011, 12:49 PM   #14
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I had a 54 Willys Overland with 3 on the tree and a Ford straight 6. It had 5.89 diffs and was simply an absurd overgear for the rig.

Max cruising speed was something around 47mph, and if 2nd gear idle wouldn't make the pull nothing else you could do would help either.

Outfitted with the largest 16" traction tires that would fit under the fender, non-power steering effort was remarkably beyond manageable, depending on how you maneuvered the steering wheel at parking speeds.
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Old 05-05-2011, 01:00 PM   #15
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I'm not familiar with the JK 6spd, my last 6spd was a Corvette LT1 and with the close ratio/close shift shaft-you could shift easy and quick and time the shift on the way up and shift going into 4000rpm and only rise a couple hundred rpm, before the clutch came out and that was a long way from "speed Shifting"

I could never Speed Shift" my '06 Xterra either-don't think you can the JK !!

I just ran #4, 20 mile run and the computer is reading 15.9 mpg-still going up !

Keepyerpowderdry

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Old 05-05-2011, 01:02 PM   #16
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Quote:
Originally Posted by MTH

That's interesting. I will say that with a manual, shifting up at 3500+ has a different feel than shifting down lower.

Once I depress the clutch at 3500+, I feel like I want to wait a moment for the revs to come back down before letting off the clutch in a higher gear. Something about the sound and feel of the engine spinning that fast without the load of the transmission makes releasing the clutch before the revs come down feel somehow . . . alarming. Especially up toward 4000 RPM. Maybe Zombie knows what I'm talking about.

Perhaps the auto overcomes this . . . or perhaps I am just making up a problem that doesn't exist.
I smell what you're stepping in

I usually pause a good full second in neutral to let the RPMs settle before completing the shift.

The 3.8 deserves more credit than I give it, yet it seems fragile (for a lack of a better word), above 3k.
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Old 05-05-2011, 03:29 PM   #17
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...265/70r17 bfg km2's (31.25"), 3.73 gear ratio, 65mph, 6th gear OD...2200 rpm's.

...i shift at around 2500 rpm's.

...i average, by hand, around 17 mpg. with all around use.

...i don't think i've ever touched 3000 on the tach.
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Old 05-06-2011, 12:59 PM   #18
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Quote:
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Once I depress the clutch at 3500+, I feel like I want to wait a moment for the revs to come back down before letting off the clutch in a higher gear. Something about the sound and feel of the engine spinning that fast without the load of the transmission makes releasing the clutch before the revs come down feel somehow . . . alarming. Especially up toward 4000 RPM.
Quote:
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I smell what you're stepping in
I usually pause a good full second in neutral to let the RPMs settle before completing the shift.
By the way Zombie, I tested our apprehension on the way to work this morning by letting off the clutch just like normal and upshifting even though I was up around 3500 RPMs. Didn't wait for the RPMs to drop at all. Made me nervous.

Guess what?

It's fine. Shifted perfectly. This is a made-up problem.

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Old 05-06-2011, 03:32 PM   #19
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Nice!

I found this video when I was looking for exhaust comparisons. I don't think I could bring myself to run the RPMs up this high, but this guy doesn't seem to mind.



Edit: might be an automatic trans
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Old 05-06-2011, 04:14 PM   #20
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I ran my 08 Sahara auto on the freeway yesterday at 70mph to get data for this thread:

Stock 255/R70-18 Bridgestone Dueler A/T (think they're 32s)

ETA: 3.73 diffs

Indicated 70mpg; tach 1800 rpm as close as I could tell. 65=shade over 1500 rpm.

I'm still averaging close to 22.5 mpg calculated at the pump, gallons divided into miles since last gas.

If I want to kill my mpg for some reason, I can run around the city all day doing jack rabbit starts and lots of idling. I was actually able to make it go down to 15.3 once, but that was enforced drudgery of city driving all day.
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Old 05-06-2011, 04:19 PM   #21
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Thats great mileage and if it's important to you, I'd never go to bigger tires and regear-you're set-


Quote:
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I ran my 08 Sahara auto on the freeway yesterday at 70mph to get data for this thread:

Stock 255/R70-18 Bridgestone Dueler A/T (think they're 32s)

Indicated 70mpg; tach 1800 rpm as close as I could tell. 65=shade over 1500 rpm.

I'm still averaging close to 22.5 mpg calculated at the pump, gallons divided into miles since last gas.

If I want to kill my mpg for some reason, I can run around the city all day doing jack rabbit starts and lots of idling. I was actually able to make it go down to 15.3 once, but that was enforced drudgery of city driving all day.
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Old 05-06-2011, 07:49 PM   #22
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the trick of course, as summer/inevitable off road sirens sing to me, whether I will choose rational DD mpg stuff, or just experiment with 'how far' a stocker can reliably go in my kind of use....

Fortunately there's a ton of prior mistakes/experiences I don't feel the need to repeat.
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Old 05-06-2011, 08:04 PM   #23
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Yea and you'll weigh the DD requirements for your playtoy !!

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the trick of course, as summer/inevitable off road sirens sing to me, whether I will choose rational DD mpg stuff, or just experiment with 'how far' a stocker can reliably go in my kind of use....

Fortunately there's a ton of prior mistakes/experiences I don't feel the need to repeat.
If I had to work anymore, I wouldn't be doing what I am to this jeep, I'd probably have other demands for my time/money/background-etc !

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Old 05-06-2011, 09:34 PM   #24
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My 11 sahara with 373 auto and the sahara 18 inch wheels, and stock tires does 1900 rpm at 70 mph as shown on my factory gps. The junk jeep speedo shows 71 mph for some reason. Makes about 20 to 22 on the highway, and 17 to 18 all around driving. The gps is probably dead on. I don't know about the tach, but it's probably pretty close!
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Old 05-08-2011, 10:17 PM   #25
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2008 Rubicon
32"
Stock Gears
70mph 2000 rpm
14.5 - 17 mpg

About to re-gear - my overdrive is a Joke - will go with 5.13
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Old 05-09-2011, 09:21 AM   #26
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Quote:
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That's interesting. I will say that with a manual, shifting up at 3500+ has a different feel than shifting down lower.

Once I depress the clutch at 3500+, I feel like I want to wait a moment for the revs to come back down before letting off the clutch in a higher gear. Something about the sound and feel of the engine spinning that fast without the load of the transmission makes releasing the clutch before the revs come down feel somehow . . . alarming. Especially up toward 4000 RPM. Maybe Zombie knows what I'm talking about.

Perhaps the auto overcomes this . . . or perhaps I am just making up a problem that doesn't exist.
I dont like that sound and feel I think I have had my Jeep at 3500 rpms once... ever
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Old 05-09-2011, 04:55 PM   #27
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2008 JK Unlimited X
32" tires off of a new Rubicon
6-spd manual.

70 mpg = 2400 rpm.
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Old 05-10-2011, 12:33 AM   #28
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2010 JKU Sahara
Manual Transmission
3.21 Rear End
Stock everything
2000 RPM @ 70 MPH in 6th
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Old 05-10-2011, 05:55 PM   #29
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Quote:
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2008 JK Unlimited X
32" tires off of a new Rubicon
6-spd manual.

70 mpg = 2400 rpm.
Oops, let me clarify:
2008 JKU X
32" tires
3.73 Gears
6-spd manual

Yields:
2400 rpm at 70 mpg.
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Old 05-10-2011, 10:08 PM   #30
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'07 JK 2dr
6 spd
33" tires
3.73 actual gear ratio Im not sure the effective w/ my tire size..
65mph puts me right at 2500 rpms in 6th.

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