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Discussion Starter · #1 ·
Flow of traffic is 80mph in the parts of SoCal I frequent. What gear ratio would be appropriate for 315/75/16 tires (should measure out around 34" mounted)?

I have a 2008 3.8L automatic with a d30 upfront. Currently at 3.73
 

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With the 3.8, you need to gear a lot deeper than the 3.6. Ideally 5.38 or 5.86!!! But the lowest that will even fit in a D30 carrier is 5.13. Since that is pretty much the "limit" and the r&p get tiny, best I would recommend is 4.88. I know some folks do 5.13, but recognize that becomes a even huger weak spot going so small.
 

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I'll go with 5.13's in your case, ideally you should go higher because of your trans ratio, but i think 5.13 is at the limit of the D30 because the space it takes inside the diff.
 

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Discussion Starter · #4 ·
Since that is pretty much the "limit" and the r&p get tiny, best I would recommend is 4.88. I know some folks do 5.13, but recognize that becomes a even huger weak spot going so small.
That's an interesting consideration. Do you have any clue on the critera for selecting the green zone on that chart? I think I might have found a tool that allows for more customization of speed and tire size, but I have no clue what RPM is optimal.
 

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I wouldn't recommend driving a jeep at 80+ period lol.
 

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We've yet to have the jeep up to or beyond 65 MPH. In other words.. avoid the beltways and take the back roads.. I deliberately built the jeep this way. My "sports car" days are over.. time for the slow ride.
 

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We've yet to have the jeep up to or beyond 65 MPH. In other words.. avoid the beltways and take the back roads.. I deliberately built the jeep this way. My "sports car" days are over.. time for the slow ride.
Even in stock form the wrangler feels sketchy at 75. Lol
 

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Discussion Starter · #8 ·
I wouldn't recommend driving a jeep at 80+ period lol.
Handles perfectly fine with the AEV kit

We've yet to have the jeep up to or beyond 65 MPH. In other words.. avoid the beltways and take the back roads.. I deliberately built the jeep this way.
We don't really have backroads here, just more freeways that are somewhat parallel
 

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Mommymallcrawler has best advice for you.

I also ride highways 80+, since that is often the flow of traffic and because there aren't back roads in NYC.

It will be an adjustment from stock to wider aftermarket tires- it can slightly more floaty effect than stock.



Sent from my iPhone using Tapatalk
 

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That's an interesting consideration. Do you have any clue on the critera for selecting the green zone on that chart? I think I might have found a tool that allows for more customization of speed and tire size, but I have no clue what RPM is optimal.
The gear chart takes the engine performance into consideration already. That is why the chart for the 3.6 is different than the 3.8. Green is basically "mallcrawler zone". Blue gets you into "performance" zone. Also consider what you do with your Jeep. Higher elevations, towing, Offroad driving, lots of hills - all the more reason to look to blue zone. Anything green or blue will be ok. And it's generally better to be geared a little lower (higher gear number, say 5.13) than higher (lower gear number, say 4.56).

I wouldn't recommend driving a jeep at 80+ period lol.
:lmao::lmao:
 

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We don't really have backroads here, just more freeways that are somewhat parallel[/QUOTE]

I concur. Backroads don't exist unless you drive 2 hours to get there.
I have a wee bit of death weaving as I like to call it. I am driving 65 MPH right now in SoCal and it's scary. First off at 65 MPH you're getting flicked off by little old ladies in Pruis' as they go around you. Not to mention the idiots that don't know how to merge and cut you off getting on the freeway...or being behind all the work trucks and semi's I can't see around.
I need to get my Jeep fixed so I can safely drive 80 in the fast lane again.
:lmao:
(Oh and sorry but I have a 3.6 and can't actually contribute any meaningful advice...)
 

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Discussion Starter · #14 ·
The gear chart takes the engine performance into consideration already.
I get that, but the chart is for 70mph, not 80mph so I was trying to find out if anyone had knowledge/experience with the latter
 

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..First off at 65 MPH you're getting flicked off by little old ladies in Pruis' as they go around you...
My experience up here in the great Northwest is the opposite. Prius drivers hold up traffic more than any other vehicle on the road these days.

Not that other types of vehicles and drivers don't, mind you, it's just that I see these ones doing it more likely than not. Could be a NW thing.
 

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I get that, but the chart is for 70mph, not 80mph so I was trying to find out if anyone had knowledge/experience with the latter
Try this:
Gear Ratio Calculator
Note that the tranny is the Dodge WA580. Though now that I think about it- shouldn't there be two listed? Ie: the 4 speed tranny with the 3.8 and the 5 speed with the 3.6? I did find it accurate for the 3.6. Grimm is on here so you can PM him (search "members")- or go to his Facebook page. The gear ratio chart is handy but it also bothered me that it's only 70 MPH that people look at. What about city as well?

Check out Robert at RWKHaus for your gear swap. One man show out of his (home) garage but gears & lockers are his thing. I doubt you'll find better pricing.
Lockers Posi LSD Gears Axles ~ Discount Drivetrain parts ~ RWK Haus Supply
 

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I get that, but the chart is for 70mph, not 80mph so I was trying to find out if anyone had knowledge/experience with the latter
Here you go:



You didn't say which tire you were running, so I went with a Cooper that has a 34.6" diameter, and 33.44" weighted rolling diameter. Other tires of that size should be very close.

Mark
 

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The gear ratio chart is handy but it also bothered me that it's only 70 MPH that people look at. What about city as well?
What speed and configuration would you like to see? The image above is from a little Excel calculator I "wrote" that'll show correct results for any JK(U) with any tire and gear combination at any speed.

Mark
 
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