Selection of Re-Gearing Shop
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JK Enthusiast
Joined: Feb 2009
Posts: 383
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From: Indianapolis, Indiana
JPop... Thanks
There isn't anything special about doing a Dana rear end as opposed to a Ford or GM. The guys you trust are a better candidate than guys that specialize in Jeep stuff.
The right approach to gear selection is first rolling on what you already have. People who suggest you need 5.13s or 5.38s didn't listen to what you had to say and where you're from. A guy in Colorado will have more of a hill to deal with getting out of his driveway than you will find in the whole state of Indiana.
Where the JK likes to be RPM-wise is dependent on the rolling weight and what is sprung and unsprung weight. Heavier tires like the Toyo MTs are going to require a little more gear than some of the tire/wheel combinations that might weigh 20-25lbs less per corner. Bumpers, gear, winch, 2/4 door all will add weigh and with added weight a little more power/RPMs to keep it rolling efficiently.
Fuel efficiency wise, (2 door, 6 speed, long flat highway commute) my Jeep ran most effectively at 18-2000 RPMs before rolling on 35s. After the 35s I need an additional 2-300 RPMs to keep it happy, but again that's on the highway flats, without the big slam auto downshift going up Colorado grades. It's a different animal and rolling on your stock 4.10s for a while will tell you where you need to be way better than someone on the other side of the country.
One last thing. When the time does come to make the gear switch I would err on the steeper side. With all that you or anyone else does to their Jeeps there aren't many weight saving measures that will ever take place and it's not a job that you want to do twice. The primary detriment is a few hundred RPM and some fuel mileage but you'll never make up for the cost if you need to do it twice.
The right approach to gear selection is first rolling on what you already have. People who suggest you need 5.13s or 5.38s didn't listen to what you had to say and where you're from. A guy in Colorado will have more of a hill to deal with getting out of his driveway than you will find in the whole state of Indiana.
Where the JK likes to be RPM-wise is dependent on the rolling weight and what is sprung and unsprung weight. Heavier tires like the Toyo MTs are going to require a little more gear than some of the tire/wheel combinations that might weigh 20-25lbs less per corner. Bumpers, gear, winch, 2/4 door all will add weigh and with added weight a little more power/RPMs to keep it rolling efficiently.
Fuel efficiency wise, (2 door, 6 speed, long flat highway commute) my Jeep ran most effectively at 18-2000 RPMs before rolling on 35s. After the 35s I need an additional 2-300 RPMs to keep it happy, but again that's on the highway flats, without the big slam auto downshift going up Colorado grades. It's a different animal and rolling on your stock 4.10s for a while will tell you where you need to be way better than someone on the other side of the country.
One last thing. When the time does come to make the gear switch I would err on the steeper side. With all that you or anyone else does to their Jeeps there aren't many weight saving measures that will ever take place and it's not a job that you want to do twice. The primary detriment is a few hundred RPM and some fuel mileage but you'll never make up for the cost if you need to do it twice.
Thanks for your input.
I may have read your post too quickly; but, I'm not sure if I understood you to say that you had re-geared after putting the 35s on, or not. If you did, what were you comfortable with?
I suspect I will end up going with 5.13s when everything is said and done. Before my rig sees its final days, I hope to be spending a lot more time in Moab... possibly even relocating. The 5.13s may be a better choice for the long term. Your suggestion on erring on the side of taller gears make sense, and it appears to be pretty consistent with a lot of the other advice that I've been given in this forum, as well as advice other posters have been given here.
(I may be the only one who thinks this way; but, my concern with a lower ratio (such as going with the 4.88 or even 4.56 gears) isn't mileage. My concern is powertrain longevity. It may not be an issue with any of the gearing options. Another reason that I was originally leaning toward the 4.88s had to do with the fact that that ratio is the next higher one up from 4.56s. 4.56s are the closest to matching OEM 4.10s when the new 35 tires are factored in; but, with the additional weight I bumped it up to 4.88. I have no issues with the OEM set-up; but, I really haven't put the JK to any kind of test yet. Being that I can't find fault with the current 4.10 set-up, it's hard to be critical of it's equivalent when the 35s are installed.)
Thanks again for your post.
Jeff
JK Junkie
Joined: Jul 2008
Posts: 3,017
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From: Lakewood, OH
I have not re-geared yet as what I felt was the right gear for me was 4.56s and they were unavailable early last summer when they would have made the most sense for me to have. The rest of the year not so much as I have a long flat highway commute, some modest wooded trail rides and the 3.73s do just fine.
The things I took care of before rolling on 35s certainly made a difference. CAI, Headers, Exhaust and programmer. I'm probably making about 10% more power than a stock 3.8. Also, a 6 speed makes a huge difference in getting the right power to the ground and there isn't the huge jump in RPMs that the auto has when downshifting.
For you and with the automatic I believe either the 4.88s or 5.13s will get you where you want to be. At highway cruise speeds you're talking about 150 rpm difference between them and it's perhaps the biggest shoulder shrug of a decision because you aren't likely to be disappointed with either.
The things I took care of before rolling on 35s certainly made a difference. CAI, Headers, Exhaust and programmer. I'm probably making about 10% more power than a stock 3.8. Also, a 6 speed makes a huge difference in getting the right power to the ground and there isn't the huge jump in RPMs that the auto has when downshifting.
For you and with the automatic I believe either the 4.88s or 5.13s will get you where you want to be. At highway cruise speeds you're talking about 150 rpm difference between them and it's perhaps the biggest shoulder shrug of a decision because you aren't likely to be disappointed with either.