chaznad

Hey all,
Have my re-gear appointment setup for next week and I'm confused as to which I should choose. I would love some real life input because charts don't factor weight of the vehicle
I would consider my JK fairly heavy with all steel armor and a winch, plus the bigger wheels and tires ( mods are in sig ) but this is a daily driver that needs to see 75-80 comfortably on the highway since I'm on the highway a lot.
I feel 4.88 is the better highway choice but 5.13 would be better for getting around town . not sure ? any input is appreciated
This is costly and I don't want to regret.

shabbernigdo

are they true 38's? i just had a similar issue but im running 35's that measure out at 34. i was worried about weight and such as well. so i was trying to decide between 4.56 and 4.88. i decided to play it safe and go with the 4.56 like most of the interned seems to suggest and like them. so ide imagine that for what you have 4.88 would be good as well like most places seem to suggest. 4.88 will put you back near what a rubicon would feel like and if you want a little more grunt go with the 5.13. Noise level changes above 3k rpm on the 3.6 so you may wanna check it out and see if it bothers you.

Biginboca

If it's for the 2007 in your sig do the 5.13. I have a 2011 with the same engine and manual tranny as the 2007. I'm daily driving 15k miles a year in South Florida and cruising the highway every day at 80-90 on the way to work. With my 37's it's perfect still has decent get up and go for a jeep I can even stay in 6th gear and pass on I95, I think with 38's and 4.88 you won't have a usable 6th gear. And for what it's worth 5.13 are amazing in 4 low too with Rubi transfer case.

If you are talking about a jeep with the newer engine then yes 4.88.

chaznad

Hey there, You go 80-90 with 5.13 and 37's in a 6 speed ? what RPM's are you running at those speeds, seems a little high , from what I read people with 37's doing 70mph will run rpm's at 2700-2800. What tires do you have ?

BTW The diameter on mine is 37.7

Biginboca

Yes. I just got back from Blue Ridge GA last week cruised 10 hours nonstop except for gas at those speeds and do all the time for the last 6 years. I'm running the STT Pros my stats are in my profile. 80 is about 3k rpm and 90 is about 3200 rpm so your figures sound right. The 3.8 likes 3k rpm, and I don't see you having the power to push 38's on a heavy JKU at less than 2800 rpm on the highway in 6th gear. The 3.8 does not have a lot of power look at the torque curve you need all the revs you can get lol

I'm getting like 9-10 mpg highway 80-90 and 12-14 city for a combined around 12 mpg. Get used to this lol....

chaznad

is it safe to be cruising at 3000-3500 at long periods of time . Ive always kept at 2800

Biginboca

I did a 2500 mile road trip over 12 days in Sept to Northern KY with wife and 2 kids loaded down luggage and gear. Same deal 80-90 all day long. Been doing this for 6 years I've got over 100k miles on this set up.... and your RPM's will be maybe 100 lower than mine you are running 38's not worn down 37's that measure maybe 35". If you like performance and cruising in 6th on the highway I would do 5.13. If you like shifting and sluggish performance with fuel economy do the 4.88. I have no regrets. Something else to consider is having lower revs in 6th might mean pushing the peddle deeper and possibly less fuel economy than at higher revs, not sure how that would play out. I know at 50-60 I get better fuel economy in 5th gear than 6th. Found that out on a drive to the keys down US1.

Ajkaz

I run 4.88's with very light 37's (Mastercraft Courser MXT) and a 6 speed on a 3.8. Its a great setup with plenty of power and torque both at high speed and low. With that being said, your 38's and wheels are substantially heavier than my 37's, the difference in rotational mass and tire circumference would likely make 4.88's very sluggish. I would look at 5.13s or 5.38s.

chaznad

well i just found a brand new set of MOTIVE GEARS in 5.13 with install kits on CL for $400 so this might just be a go . Based on what everyone is saying I think it might be the best route . As long as i can do 80 on the highway with zero issues I'm happy. I obviously prefer the peppiness around town, just concerned about the highway
Do you guys even use 1st gear , feel like it will be short lived

Biginboca

Yes I use first gear, I like my clutch to last as long as possible lol

All kidding aside with 5.13 and 37's my Rubicon feels very close to how it did stock with 4.10 gears (meaning the engine, throttle, and driveability of it.).

*Edit: No surprise I just checked the gear chart and at 70 it's only turning 200 revs more than a stock MT Rubicon would (~2530 vs 2740.). Yours on 38's would be even closer at like 2670 revs.