Got some 35's, How are my RPM's? - Rubicon Owners Forum
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post #1 of 18 (permalink) Old 05-25-2019, 05:39 PM Thread Starter
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Got some 35's, How are my RPM's?

I upgraded to 35x12.5x15 BFG's. I still have the stock 4.10 gears on my 2004 Rubi. It rides fine and doesnt feel too different than with the 33's I had before.

Question is, are my RPM's in a good place at 65mpg and 70mpg, see images.

Thanks...
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-Mojorisen

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post #2 of 18 (permalink) Old 05-25-2019, 07:04 PM
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It depends on where you drive and your personal preference. Mine was a dog so I re-geared. I couldn't imagine being able to be in fifth gear very often at those RPMs. This 4.0 engine develops it's power and likes being run between 2.5 and 3.5K RPMs.
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post #3 of 18 (permalink) Old 05-25-2019, 07:18 PM
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65 and 70 miles per gallon .... holy crap!!! haha . Do you have an AT or MT transmission? Those seem low if you're in 5/6th gear or over drive. I wouldn't want to be in the Mountains with those rpms. I have "33's" with the stock 4.10 gears and 5speed and it's tolerable but once I get into the mountain roads of Colorado it becomes a real dog and I often found myself in 3rd gear just trying to keep up with highway traffic. I've thought about regearing to 4.56 but am holding off until next year cause I might just pull the trigger and go to 35's and be done with it.

I'd personally look into regearing. I usually shift around 2.5k rpm's for smoother shifts and the engine seems happier when it's at or just above 2k cruising around. It may not be the most fuel efficient way to drive it but when my 4cyl honda feels like it has more umph than my heep it's a sad feeling haha.

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post #4 of 18 (permalink) Old 05-25-2019, 08:17 PM Thread Starter
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I have an AT. Those rpm's are on flat road. On hills rpms spike as it downshifts as expected. I am thinking of jumping to 4.56 gears or 4.88. But 4.56 seems like it would be a sweet spot and 4.88 may be a hair too much. I guess it depends on the weight of the Jeep too is what i hear.

-Mojorisen

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post #5 of 18 (permalink) Old 05-25-2019, 10:23 PM
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Go with at least with 4.88, but for an auto 5.13 would be ideal, that what I (and most of the crowd running 35s on an auto) have.
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post #6 of 18 (permalink) Old 05-25-2019, 10:42 PM
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It's personal preference to a large degree. With the auto I'd do 5.13 or 5.37, myself. Have never regretted 4.88 with 35s and six speed. Which is closer to 5.37 with the auto. And I routinely run 80mph on the interstate.

- DAA
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post #7 of 18 (permalink) Old 05-25-2019, 11:06 PM
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Quote:
Originally Posted by Mojorisen View Post
I upgraded to 35x12.5x15 BFG's. I still have the stock 4.10 gears on my 2004 Rubi. It rides fine and doesnt feel too different than with the 33's I had before.

Question is, are my RPM's in a good place at 65mpg and 70mpg, see images.

Thanks...
Have the speedometer readings been corrected for the tire change(s) ?
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post #8 of 18 (permalink) Old 05-25-2019, 11:36 PM
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Back to the original question, are the RPM's good? No, that's terrible. Just keep the OD turned off for now. That's what I did for the first year or so when I went from 33 to 35's. I eventually went to 5.13 to be able to use OD again, much better now. I believe 2500 rpm is 73 mph.
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post #9 of 18 (permalink) Old 05-26-2019, 10:54 AM
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Quote:
Originally Posted by Mojorisen View Post
I have an AT. Those rpm's are on flat road. On hills rpms spike as it downshifts as expected. I am thinking of jumping to 4.56 gears or 4.88. But 4.56 seems like it would be a sweet spot and 4.88 may be a hair too much. I guess it depends on the weight of the Jeep too is what i hear.
As stated by others... with an Auto tranny and 35's, get 5.13 gears.
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post #10 of 18 (permalink) Old 05-26-2019, 08:10 PM
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Quote:
Originally Posted by Mojorisen View Post
I upgraded to 35x12.5x15 BFG's. I still have the stock 4.10 gears on my 2004 Rubi. It rides fine and doesnt feel too different than with the 33's I had before.

Question is, are my RPM's in a good place at 65mpg and 70mpg, see images.

Thanks...
Too low, The oil system on the 4.0 is ancient and was designed when engines were spinning at 3000 regularly with 28 inch tires and 4.27 geas. Spinning that engine a bit faster is better and if you don't regularly get it to 4000 it can become a problem with stuck lifters, valves and pushrods wearing rockers or stuck valves . The valves don't start to rotate until 3000 rpm, and can cause some stuck or burned valves if they arnt rotating.

peak torque on the 4.0 is around 3200-3400, 75- 80% of that should be cruise RPM 2500-2700 would be my target.

4.88 at a minimum 5.13s better.
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