4.56 vs 4.88 gearing
Thread Starter
JK Newbie
Joined: Aug 2010
Posts: 62
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From: SouthWest Virginia
4.56 vs 4.88 gearing
The JKU is equiped with ARB bumpers, winch, hardtop, full rack, and 33 in tires ( actually they are 285-70X17" so not quite 33")
I drive 80 miles / day on the interstate, things were ok unitil I added the rack, now th JK can't get out of its own way.
Since a diesel conversation has not materialized so I am going the regear route.
This is an automatic with 4.10 gearing. on a one for one basis 4.56 gearing would bring back operating rpm to stock , roughly 2150rpm @ 70mph
My concern is not many people use 4.56 gears instead they go with 4.88's
WHY?
Also I plan on towing a Conqueror Compact trailer (2000lbs) out to Chaco Canyon, NM in June so the mileage performance is a big concern on a 3000 mile trip
Thoughts
I drive 80 miles / day on the interstate, things were ok unitil I added the rack, now th JK can't get out of its own way.
Since a diesel conversation has not materialized so I am going the regear route.
This is an automatic with 4.10 gearing. on a one for one basis 4.56 gearing would bring back operating rpm to stock , roughly 2150rpm @ 70mph
My concern is not many people use 4.56 gears instead they go with 4.88's
WHY?
Also I plan on towing a Conqueror Compact trailer (2000lbs) out to Chaco Canyon, NM in June so the mileage performance is a big concern on a 3000 mile trip
Thoughts
JK Freak
Joined: Feb 2010
Posts: 594
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From: British Columbia
Mine's an auto too. I haven't regeared and won't be able to afford it any time soon. If I could do it today I'm sure I'd go with the 4.88 gears. I figure with an auto they'll perform close to the 4.56. But then, I'm no expert... 
JK Jedi Master
Joined: Jan 2008
Posts: 11,463
Likes: 165
From: Austin <--> Colorado Springs
My concern is not many people use 4.56 gears instead they go with 4.88's
WHY?
WHY?
And where the auto is concerned, 4.56's are a complete waste of money.
Thread Starter
JK Newbie
Joined: Aug 2010
Posts: 62
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From: SouthWest Virginia
X2 4.56s would be a complete waste.
Unless you do 80+ mph everywhere on the highway, you want 5.13s with an automatic non-Rubi for anything 33" tires and up. If you regularly go 80-85 mph, you may want 4.88s instead of 5.13s.
5.13s and 33s would put your rpms about the same as a stock 6 speed Rubi with stock tires.
You should shoot for stock Rubi 6 speed rpms instead of stock automatic rpms.
Unless you do 80+ mph everywhere on the highway, you want 5.13s with an automatic non-Rubi for anything 33" tires and up. If you regularly go 80-85 mph, you may want 4.88s instead of 5.13s.
5.13s and 33s would put your rpms about the same as a stock 6 speed Rubi with stock tires.
You should shoot for stock Rubi 6 speed rpms instead of stock automatic rpms.
JK Newbie
Joined: Jan 2011
Posts: 17
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From: Chapel Hill NC
I have a new 2010 6 speed with a dealer installed lift kit and 35" wheels. This car from 4th -6th gear is a dog. I have never had a car I had to downshift on a highway to manage a very light grade. I read about the 4.88s but wondered about the 5.13s
I don't need to crusie at 80 plus but I would like to travel on the highway at 65-70 and be able to pass another car in less than 100 miles it might take me today to build up speed
I don't need to crusie at 80 plus but I would like to travel on the highway at 65-70 and be able to pass another car in less than 100 miles it might take me today to build up speed
JK Jedi Master
Joined: Jan 2008
Posts: 11,463
Likes: 165
From: Austin <--> Colorado Springs
Muleshoer - there is a great RPM chart in the Drivetrain FAQ's, stickied to the top of this modified area. Plug some numbers in and see what you see.
But, auto, 5.13's, 32" tires (since your 33's aren't actually 33 inches tall)
= 2600 RPM @ 70
As noted above, unless you spend all of your time at 80+mph, this is a good ratio, as it will keep the HP's up enough to compensate for the drag of bigger tires/roof rack/ect, and keep the transmission from downshifting all the time.
I ran this setup for about 6 months until I went to 35's. It was great for everything except OD/Off towing, where the rpm's climbed quite a bit. Not an issue with lighter loads, but I tow a ski boat that weights in right at the 3500lb tow limit, and prefer to tow OD off to save the transmission.
Sevangelista - you have a manual. The gearing is completely different, and will not help the OP make his decision. But yes, 4.88's are very popular for 6speeds running 35's. For higher speed driving, they will be a better choice for you than 5.13's. Just plug the numbers into the chart and see where you want to be.
But, auto, 5.13's, 32" tires (since your 33's aren't actually 33 inches tall)
= 2600 RPM @ 70
As noted above, unless you spend all of your time at 80+mph, this is a good ratio, as it will keep the HP's up enough to compensate for the drag of bigger tires/roof rack/ect, and keep the transmission from downshifting all the time.
I ran this setup for about 6 months until I went to 35's. It was great for everything except OD/Off towing, where the rpm's climbed quite a bit. Not an issue with lighter loads, but I tow a ski boat that weights in right at the 3500lb tow limit, and prefer to tow OD off to save the transmission.
Sevangelista - you have a manual. The gearing is completely different, and will not help the OP make his decision. But yes, 4.88's are very popular for 6speeds running 35's. For higher speed driving, they will be a better choice for you than 5.13's. Just plug the numbers into the chart and see where you want to be.
Last edited by nthinuf; Jan 25, 2011 at 03:06 PM.
Thread Starter
JK Newbie
Joined: Aug 2010
Posts: 62
Likes: 0
From: SouthWest Virginia
[QUOTE=nthinuf;2056501]Muleshoer - there is a great RPM chart in the Drivetrain FAQ's, stickied to the top of this modified area. Plug some numbers in and see what you see.
But, auto, 5.13's, 32" tires (since your 33's aren't actually 33 inches tall)
= 2600 RPM @ 70
Nthinuf
I will go play with the chart
So it begs the question what is the optimum performance band of a stock 3.8? understanding automatics rob energy
Thanks
But, auto, 5.13's, 32" tires (since your 33's aren't actually 33 inches tall)
= 2600 RPM @ 70
Nthinuf
I will go play with the chart
So it begs the question what is the optimum performance band of a stock 3.8? understanding automatics rob energy
Thanks