Better RPMs for gas mileage? - Page 3 - JeepForum.com

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post #31 of 59 Old 03-29-2010, 07:38 PM
13mx
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So what would be ideal for my jeep? I have 32in tires, 3.73 gears and no heavy equipment. Should I be shifting gears right at 2k, or take it to the mid 2k?

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post #32 of 59 Old 03-29-2010, 08:00 PM
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So what would be ideal for my jeep? I have 32in tires, 3.73 gears and no heavy equipment. Should I be shifting gears right at 2k, or take it to the mid 2k?
Nobody can answer that for you. Just do what feels right. Either way, 2k is a little low IMO. 2.5-3k is where I USUALLY shift from 1-3 with great results. After that, you have enough momentum that 2k is fine.

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post #33 of 59 Old 03-29-2010, 08:42 PM
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Quote:
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So what would be ideal for my jeep? I have 32in tires, 3.73 gears and no heavy equipment. Should I be shifting gears right at 2k, or take it to the mid 2k?
Try a tank one way, then a tank the other way. That's really the only way to figure out which will be better for you.
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post #34 of 59 Old 03-29-2010, 08:43 PM
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Also when you compare make sure to take into consideration if you drove on hills and what not
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post #35 of 59 Old 03-30-2010, 08:43 AM
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Exactly. Setups on smaller tires like yours are very different than larger tires. You are perfectly geared for 31's, which are quite a bit smaller than 35's. They weigh less, they are thinner, and the diameter is smaller. You can run 2000 RPM @ 65mph with no problems and good mileage. Step up to 35's and try to run 2000 RPM's at the same speed. It's a no go. Good mileage on 35's doesn't come until the mid 2k RPM range from my testing.
All the data was collected on my last jeep. 33's, 3.73s, and the 5 speed. It was in serious need of more gear.
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post #36 of 59 Old 03-30-2010, 09:04 AM
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so how does all of this relate to the auto trans? i'm thinking that it doesnt relate at all and i'm getting into something deeper than i would really like to.
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post #37 of 59 Old 03-30-2010, 11:34 AM
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This is all sort of making sense to me. Being that my TJ is back in Texas and I'm in Germany I can't test the theories. I'm running 31X10.5X15's on a 4.0 running factory gears on a D35 in the rear. What's the best mileage you've had on 31's in what gear? Assuming you run 31's.

Not much motion in the ocean, but I'm the captain.
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post #38 of 59 Old 03-30-2010, 11:39 AM
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My TJ with 31's and 3.07 gears gets its best mpg at 1,800-2,000 rpm.
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post #39 of 59 Old 03-30-2010, 11:51 AM
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Originally Posted by sxysouthtx View Post
This is all sort of making sense to me. Being that my TJ is back in Texas and I'm in Germany I can't test the theories. I'm running 31X10.5X15's on a 4.0 running factory gears on a D35 in the rear. What's the best mileage you've had on 31's in what gear? Assuming you run 31's.
Factory gears means nothing. 3.07 or 3.73? You also didn't mention auto or manual. With 3.73, you're golden with 31's. With 3.07's, not so much. I hated that combination with a passion.

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post #40 of 59 Old 03-30-2010, 12:02 PM
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Not quite sure what I have as far as gears. But it's a 5 speed manual.

Not much motion in the ocean, but I'm the captain.
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post #41 of 59 Old 03-30-2010, 12:06 PM
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Not quite sure what I have as far as gears. But it's a 5 speed manual.
When you get home from Germany, look at the 9 o'clock position of the rear diff. for a tag that says what gears you have.

Follow what this link says.

http://www.jeepforum.com/forum/f9/fa...-ratio-615380/
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post #42 of 59 Old 03-30-2010, 12:08 PM
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Appreciate it. I needed to know, and it was killing me.

Not much motion in the ocean, but I'm the captain.
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post #43 of 59 Old 03-30-2010, 12:28 PM
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If 5th gear feels like it bogs down the engine, you probably have 3.07 gears. 3.73 gears will feel much more usable.

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post #44 of 59 Old 03-30-2010, 12:35 PM
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What about 31's with an auto and 3.07 gears. That is what my friend has and it seems that the 3 speed auto helps with 3.07's.
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post #45 of 59 Old 03-30-2010, 12:41 PM
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I didn't take the time to read all the responses, but one of the best ways to get the best milage possible is to have a vacuum gauge, and then drive to maintain the highest vacuum possible. Engine vacuum is basically measure the load the engine is under. When you minimize the load (hi vacuum) your milage will be optimized. You'll be amazed how just a little throttle will lower vacuum.
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