Better RPMs for gas mileage? - Page 4 - JeepForum.com

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post #46 of 59 Old 03-30-2010, 01:16 PM
sxysouthtx
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Yeah, 5th bogs me down pretty bad unless I'm doing about 70 or 75. Kind of makes sense now. What do you think that 3.07's, 5 speed , 4.0 ,and running 31's will get me?


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post #47 of 59 Old 03-30-2010, 01:18 PM
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Yeah, 5th bogs me down pretty bad unless I'm doing about 70 or 75. Kind of makes sense now. What do you think that 3.07's, 5 speed and running 31's will get me?
I used to run the same setup--you have the NV3550 by the way. I couldn't do better than 14-15mpg and 5th was pointless. I got better mileage in 4th on the highway due to less engine load. And believe me, 5th gear isn't doing you well at 70+ either. At those speeds, you need more RPM's than that. With 4.88's and 35's I now get considerably better mileage and the jeep is MUCH more lively.

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post #48 of 59 Old 03-30-2010, 01:20 PM
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I figured that was around what I was getting. If there wasn't any other reason to regear this would be it. Thanks for the help.

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post #49 of 59 Old 03-30-2010, 01:24 PM
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I have 3.07's and 31's and I get 14MPG at the best and 12.5 mpg normal without really taking too much consideration to taking it easy on the pedal. ...Typical normal driving. I did realize I had a habit of shifting early (at low RPMS) so I have been more aware of shifting at higher RPMs lately...Ill see if that makes a difference.
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post #50 of 59 Old 03-30-2010, 01:51 PM
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I used to run the same setup--you have the NV3550 by the way. I couldn't do better than 14-15mpg and 5th was pointless. I got better mileage in 4th on the highway due to less engine load. And believe me, 5th gear isn't doing you well at 70+ either. At those speeds, you need more RPM's than that. With 4.88's and 35's I now get considerably better mileage and the jeep is MUCH more lively.
How much body armor, etc were you carrying as added weight over a stock TJ?
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post #51 of 59 Old 03-30-2010, 02:01 PM
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How much body armor, etc were you carrying as added weight over a stock TJ?
Jeep was pretty light. Stock belly skid, gas tank skid. Just rocker guards. It's a little heavier now--Rokmen gas tank skid and belly skid. But no back seat. So basically, negligible. 3.07's are just too high. 3.73 gears would have been perfect. If I remember right, I was at about 1800 RPM's @ 65mph with 3.07's and 31's in 5th. Downshift to 4th and I was at a much nicer 2250 RPM or so. To compare, I'm now at 2500 RPM @ 65mph in 5th gear.

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post #52 of 59 Old 03-30-2010, 02:04 PM
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Originally Posted by JrJmisn View Post
I have 3.07's and 31's and I get 14MPG at the best and 12.5 mpg normal without really taking too much consideration to taking it easy on the pedal. ...Typical normal driving. I did realize I had a habit of shifting early (at low RPMS) so I have been more aware of shifting at higher RPMs lately...Ill see if that makes a difference.
I'm surprised your mileage was so awful. I have 3.07, manual, 4.0L, with 30.6" (245/75/16s) tires and I'm getting 18 average. I do drive on the highway quite a bit, but still...
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post #53 of 59 Old 03-30-2010, 02:22 PM
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Originally Posted by Imped View Post
Jeep was pretty light. Stock belly skid, gas tank skid. Just rocker guards. It's a little heavier now--Rokmen gas tank skid and belly skid. But no back seat. So basically, negligible. 3.07's are just too high. 3.73 gears would have been perfect. If I remember right, I was at about 1800 RPM's @ 65mph with 3.07's and 31's in 5th. Downshift to 4th and I was at a much nicer 2250 RPM or so. To compare, I'm now at 2500 RPM @ 65mph in 5th gear.
I'm amazed at the variation in results. I'm not deluded enough to think that all TJ's will perform the same, but some people get 20+ mpg with 31's and 3.07's and others can't break 15 mpg with the same set up. Could the spark plug rail make that kind of difference (it's the only change that I know of, that might have an impact)?

EDIT: Could it be the AX15 vs NV3550?
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post #54 of 59 Old 03-30-2010, 02:29 PM
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I'm amazed at the variation in results. I'm not deluded enough to think that all TJ's will perform the same, but some people get 20+ mpg with 31's and 3.07's and others can't break 15 mpg with the same set up. Could the spark plug rail make that kind of difference (it's the only change that I know of, that might have an impact)?
I doubt that makes any difference. I could probably do a little better knowing what I know now. I drive differently than I used to. But regardless, it was a terrible combination. The jeep always felt like it was starving. Shifts were massively tall. I felt like I was always under 2k RPM. Now, it's effortless. The Jeep never starves and can cruise with plenty of power @ 35mph in 4th gear.

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post #55 of 59 Old 03-30-2010, 02:30 PM
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Originally Posted by dualtjs View Post
I'm amazed at the variation in results. I'm not deluded enough to think that all TJ's will perform the same, but some people get 20+ mpg with 31's and 3.07's and others can't break 15 mpg with the same set up. Could the spark plug rail make that kind of difference (it's the only change that I know of, that might have an impact)?

EDIT: Could it be the AX15 vs NV3550?
i would think the driving style, state of disrepair and tread pattern would make the most significant difference. mud tires vs street tires, dirty air filter, clogged O2 sensors, clogged cats, bad plugs, heavy foot, amount of city/highway driving, hypermiling, etc.
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post #56 of 59 Old 03-30-2010, 02:43 PM
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Stock 2006 Rubicon, 6 speed. Just bought it with 6800 miles on it. Best tank so far has been 15.3 mpg. I think my CJ7 does better than that.

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post #57 of 59 Old 03-30-2010, 03:22 PM
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Originally Posted by Unlimited04 View Post
i would think the driving style, state of disrepair and tread pattern would make the most significant difference. mud tires vs street tires, dirty air filter, clogged O2 sensors, clogged cats, bad plugs, heavy foot, amount of city/highway driving, hypermiling, etc.
Thought I saw a pattern (well as much of a pattern as you can see in 4 total cases), where the 97-98 TJ's had better mpg with the 3.07/31 combo, than the 03-05 TJ's. Not enough to be meaningful, just enough to think there may be a pattern evolving.
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post #58 of 59 Old 03-30-2010, 05:02 PM
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I wouldnt think that years would make a difference, maybe you didnt take into account transmissions
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post #59 of 59 Old 03-30-2010, 06:00 PM
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I'm not sure how you would take into account tranny's unless you could set up a large controlled test to see what impact they would have (probably need road and dyno, to really assess), but I did pose the question (look at post #53).
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