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Unread 03-28-2010, 04:05 PM   #1
MattSink
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Better RPMs for gas mileage?

Do you get better gas mileage going 60 mph in 5th gear at ~1850 rpm, or in 4th gear at ~2250 rpm? I know usually the lower rpm is better, but it seems like the engine is working harder at the lower rpms.

BTW, 2000 TJ 4.0 5 speed, 32" tires, 3.73s

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Unread 03-28-2010, 04:09 PM   #2
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I never could get better or worse mileage from changing driving style on any jeep or dodge product. highway at 80 or town at 35 is always the same or close enough not to matter.

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Unread 03-28-2010, 04:10 PM   #3
pielet97
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Complicated question and the only true answer is if you figure it out on your own jeep.

Empty the tank, put in 1 gallon measure distance. Repeat in the other gear.

Anyone who says the know the answer is either wrong or a jeep engineer.
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Unread 03-28-2010, 04:13 PM   #4
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Quote:
Originally Posted by pielet97 View Post
Complicated question and the only true answer is if you figure it out on your own jeep.

Empty the tank, put in 1 gallon measure distance. Repeat in the other gear.

Anyone who says the know the answer is either wrong or a jeep engineer.
X2. This is a great way to find out. Just make sure not to forget the jerry can to refill the tank when that original 1 gallon runs out!
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Unread 03-28-2010, 04:17 PM   #5
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Your thought process is correct. 4th gear in your setup will give you better mileage since it's not lugging the engine. I manage 17-18mpg avg. on 4.88's, 35's and the NV3550 and am always in the 2-3k range. Don't lug the engine and it will love you. To prove my point, I had a 100 mile trip to make today and wanted to see what my avg. mileage would be. Running 55-60 mph almost the whole time (about 2-2.25k RPM) I managed 19.3mpg (miles driven/gallons used). I couldn't do that if my life depended on it when I had 3.07's and 31's.
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Unread 03-28-2010, 04:23 PM   #6
MattSink
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Thanks Imped, I was taught that low engine speed = less gas being used, but I figured there would be a point where it is too low to operate efficiently.
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Unread 03-28-2010, 04:26 PM   #7
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I've recorded every fill up since I bought it in 2008. I should crunch the numbers.
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Unread 03-28-2010, 04:27 PM   #8
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Thanks Imped, I was taught that low engine speed = less gas being used, but I figured there would be a point where it is too low to operate efficiently.
Yep. Anyone thinking that isn't considering everything. There's just more to it than RPM's. A jeep traveling at 65mph takes much more energy to move efficiently than a Civic moving at 65mph because its shape. 1800 RPM just isn't enough and the engine will be working overtime. Come up to the 2300-2700 range and it's much happier.
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Unread 03-28-2010, 04:30 PM   #9
pielet97
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Quote:
Originally Posted by Imped View Post
Your thought process is correct. 4th gear in your setup will give you better mileage since it's not lugging the engine. I manage 17-18mpg avg. on 4.88's, 35's and the NV3550 and am always in the 2-3k range. Don't lug the engine and it will love you. To prove my point, I had a 100 mile trip to make today and wanted to see what my avg. mileage would be. Running 55-60 mph almost the whole time (about 2-2.25k RPM) I managed 19.3mpg (miles driven/gallons used). I couldn't do that if my life depended on it when I had 3.07's and 31's.
Well considering your experience isn't with 32's or 3.73, you must be a jeep engineer
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Unread 03-28-2010, 04:34 PM   #10
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I have one of those Scan Gauge II and you can program it for current fuel mileage (it updates every 3 seconds I think) I have noticed at 45mph on level ground 6th gear is the best mileage, but if its slightly hilly then I drop it back to 5th and get better mileage. Now keep in mind I'm in central Indiana so hilly for us is just bumps to a lot of other people and the 6 speed is obviously geared different.
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Unread 03-28-2010, 04:35 PM   #11
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Originally Posted by pielet97 View Post
Well considering your experience isn't with 32's or 3.73, you must be a jeep engineer


32's and 3.73 gears is definitely more ideal than 31's and 3.07's, about 20% lower. And I've dealt with enough tire/gear combinations and have ran the numbers more than enough times--I'm generally right when it comes to this subject.
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Unread 03-29-2010, 02:30 AM   #12
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Quote:
Originally Posted by Imped View Post
Your thought process is correct. 4th gear in your setup will give you better mileage since it's not lugging the engine. I manage 17-18mpg avg. on 4.88's, 35's and the NV3550 and am always in the 2-3k range. Don't lug the engine and it will love you. To prove my point, I had a 100 mile trip to make today and wanted to see what my avg. mileage would be. Running 55-60 mph almost the whole time (about 2-2.25k RPM) I managed 19.3mpg (miles driven/gallons used). I couldn't do that if my life depended on it when I had 3.07's and 31's.
Quote:
Originally Posted by Imped View Post


32's and 3.73 gears is definitely more ideal than 31's and 3.07's, about 20% lower. And I've dealt with enough tire/gear combinations and have ran the numbers more than enough times--I'm generally right when it comes to this subject.
Imped's right 100%. 3.07 gears and 31" tires suck. Ask me how I know. To put it in perspective, I run darn near the same mileage in my Jeep that now has 5:13 gearing, 35's, much more armor, equipment, beefier axles, bigger, heavier wheels...(the list goes on) than I did when I was with the stock 3.07 and 31's with no heavy equipment or options. That should be an indicator as to how bad 31's with 3.07 gears are.
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Unread 03-29-2010, 02:37 AM   #13
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Quote:
Originally Posted by Imped View Post


32's and 3.73 gears is definitely more ideal than 31's and 3.07's, about 20% lower. And I've dealt with enough tire/gear combinations and have ran the numbers more than enough times--I'm generally right when it comes to this subject.
I also concur with Imped, I ran 3.07 on 33's and it seemed when I lugged the engine less and went to a lower gear, I did get better gas mileage. It seems counterproductive, but heck it works.
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Unread 03-29-2010, 05:43 AM   #14
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Look around the forums on ecomodder.com. They generally say lower RPM is better, but don't take into account all the funky things you can do with a Jeep's ratios. There is plenty of advice on there for how to accurately calculate your MPG, and driving techniques to make the most of what you have.
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Unread 03-29-2010, 06:53 AM   #15
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Originally Posted by JEK3 View Post
Look around the forums on ecomodder.com...
That web site is dedicated to vehicles with very little in common with Jeeps. They talk about narrow over inflated tires, wheel covers, improving aerodynamics, ... I don't see them adressing shoving a 4000 lb brick on agressive tread tires down the road. They aren't running vehicles with transfer cases, multiple drive shafts and axles,...
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