I guess that's one of the multiple reasons why this forum exist..

 

For those interested, a handy little video posted on youtube care of 4wd.com
http://www.youtube.com/watch?v=AWbQpsLMDlQ

 

i have a 2 doors jk sahara with dana 30 (front) and dana 44(rear) with 34" tires mi gas milage is very expensive so i want change my gears, is it ok if a buy 5.13 for both axels or is better if a put on dana 30 4.88 and dana 44 5.13...?

 

OW

Pick one diff ratio, and use it for both diffs.

If the front/rear are different, they will be turning the tires at different SPEEDS.

YOU NEED the front and rear of the Jeep to go the SAME SPEED...otherwise, the front are rear will arrive where you were going at different times.



A 4.88 in front, and a 5.13 in the rear, will mean the front is turning FASTER, dragging the rear tires along skidding and skittering as needed.

In 1 mile for example, the front would go the mile, but the rear would go ~ 5% less

So, the front of the jeep goes 5,280', but the rear only goes ~ 5,022'

You jeep STARTED the trip a dozen or so feet long, but, would need to be ~ 257' long at the end of one mile.



Assuming the jeep stayed the same length - you would have skidded a total of ~ 257' to go ONE MILE.


Obviously, the jeep would stay the same size, but, the front tires would wear out faster from the extra drag, the rear tires would wear out faster from the extra scuffing, etc, and your MPG would be crap from all the combined extra resistance, etc.

Handling would be odd too, as you'd potentially trigger trailing throttle induced over steer type symptoms.

The Brakes should work fine though....and should be used instead of the gas in the above scenario.







And so forth.



So, get the 5.13's front and rear if that helps.

 

Please clarify. Automatic 08 JK with 373's going to 513's. Do I need new carrier for front only, back only or both? Talked with 3 different suppliers and got 3 different answers. Thanks

 

Read the thread again, first page...I know it's a lot of info but to clarify again. ONLY those with 3.21 gearing require a new carrier in the front D30 ONLY in order to regear to numerically higher ratios. Again...only 3.21 ratio D30 carriers require replacement with a carrier capable of carrying 3.73 and higher. ALL JK rear 44's already utilize a carrier of 3.73 and Down, therefore feel free to keep what you have regardless of the gear ratio you choose, or replace with a locker of your choice with 3.73 and Down rating...yes, even if you are installing 5.13's, get the 3.73 an down carrier. If any dealer, vendor, installer is giving you info other than this find someone else as they are not familiar with the JK. If in doubt, call Dave at Northridge4x4 please.

 

Thanks. I did read the entire write up, several times, which is well written. Having been told different answers I just thought it best to have it clarified before I ordered. Thanks again....

 

Understandable. I believe due to the JK's wider stance and hefy weight the engineers wanted to address some driveline strength concerns, hence some changes in the pinion and gearing sizes. The D44 pinion is much more robust compared to previous D44 applications. The JK D44 also uses a "thick" ring gear...literally the ring gear is thicker than previous designs which is why for proper gear installation in JK D44's the carrier must be a 3.73 and Down shouldered unit. This is also why the emphasis is that you must obtain JK specific gears in order to do a gear swap.
Also, to make mention on the D30, it is of a reverse rotation design as opposed to the TJ's previous standard rotation design and similar to the YJ's reverse rotation, but unlike the YJ our D30 pinion uses a crush sleeve instead of shims to set pinion pre-load. Also, the D30 does not utilize a "thick" gear design which necessetates the change in carriers to a 3.73 and Up unit only for those with 3.21's.
Due to these changes not all suppliers, vendors, mechanics etc are up to date with respects to the Jeep JK, which is why I suggest contacting reputable vendors "in-the-know" like Northridge4x4 who have an intimate knowledge of the Jeep JK.
I hope this helps.

 

cheers for all that haven't been on in awhile. sweet

 

This is awesome!