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Old 07-14-2019, 12:09 AM
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Can't decide 4.56 or 4.88 on 35x12.5

I'm really unsure if I should go 4.56 or 4.88 on my 2012 3.6 auto (w5a580). I live at 1200ft, drive my jeep as a DD, mild offroading (overland) a couple times a year (ocassionally at 5000ft), but highway speed limit is 80mph and the view is non-existent, so driving 85 is ideal for my sanity. I tried using the RPM calculator with trans ratio of 0.83 (got this OD ratio on interwebs somewhere) and get a RPM of 3353 at 85 with 4.88 and RPM of 3133 at 85 with 4.56. I know it's only 250rpm difference, but when nearing 50% of readline it seems high to be running engine at 3353 for hours on end. I don't tow, but if I were to it would be two or three dirt bikes on a small trailer (maybe 1.0-1.2k lbs?) And that would be once maybe twice a year and I'd likely keep at 75-80mph pulling anything. Your thoughts?

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Old 07-14-2019, 08:15 AM   #2
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There are a million threads on this and basically you canít go wrong with either. I figured since I was going to spend the money I was gonna go 4.88s, I didnít want to regret 4.56s.

I just drove 10 hours to the beach and didnít even notice the extra rpms. Youíll be happy either way. I only think the people who tow or drive at high elevations regret the 4.56 and those are specific situations.


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Old 07-14-2019, 08:46 AM   #3
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4.88s.
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Old 07-14-2019, 09:20 PM
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Wow, really that simple. I know there are a ton of threads on gear selection. So sorry, the first bunch I read through didn't really include high speed cruising. Do those calculator numbers come out about right though? 85 at 3300 RPM or so?
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Old 07-14-2019, 09:29 PM   #5
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Originally Posted by JeepJeepBeepBeep View Post
Wow, really that simple. I know there are a ton of threads on gear selection. So sorry, the first bunch I read through didn't really include high speed cruising. Do those calculator numbers come out about right though? 85 at 3300 RPM or so?
The biggest uncertainty is true tire rolling diameter, ie 35s may actual be 34, resulting in higher rpms at a given speed. Other than that it's based on gear ratios, so unless you've got gears slipping, it'll be accurate.
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Old 07-14-2019, 10:46 PM
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Originally Posted by JeepJeepBeepBeep View Post
Wow, really that simple. I know there are a ton of threads on gear selection. So sorry, the first bunch I read through didn't really include high speed cruising. Do those calculator numbers come out about right though? 85 at 3300 RPM or so?
The biggest uncertainty is true tire rolling diameter, ie 35s may actual be 34, resulting in higher rpms at a given speed. Other than that it's based on gear ratios, so unless you've got gears slipping, it'll be accurate.
Yeah, I used the manufacturer's specified true dimensions, which matched what I measured in person as well; 34.75"
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Old 07-15-2019, 09:18 AM   #7
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If you do a lot of high speed interstate driving at lower altitudes and you don't tow, I would go with 4.56. Especially with only mild off roading.

Running your engine at 3,000 RPM won't kill it. Modern engines can handle that sustained speed all day long.

Running at 85 MPH will decimate your gas mileage no matter what gears you run so that's not as much of a factor.

Given your use of the Jeep, 4.88s would be more gear than you need.

A case could even be made for 4.10s in your situation.

If you have 3.21s now, definitely upgrade to better gears for 35's.
If you have 3.73s, go to 4.56s. Going up to 4.10s from 3.73s isn't a big enough bump to be worth the money.
If you have 4.10s, there isn't a whole lot of reason to change them.
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Old 07-15-2019, 03:19 PM   #8
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I'd go with 4.88's.

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