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-   -   rear axle gears (http://www.jeepgarage.org/f73/rear-axle-gears-44946.html)

trelos78 07-20-2012 06:19 AM

rear axle gears
 
Has anyone tried changing the rear axle gear ratio on a 12 GC Hemi Overland? Looking to go from 3.45 to 3.70. Some have told me I have to change both front and back others said only rear is fine. Does anyone know how much something llike this would cost? Any feedback greatly appreciated!!!!

tnjeep 07-20-2012 06:38 AM

Re: rear axle gears
 
You have to have the front and rear ratios matching if you have a 4wd. If you got the 2wd version then just the rear has gears, the front does not. If I was spending the dough to change gears I would think about going closer to the 3.90 area if the stock is 3.45 maybe even closer to 4.10 if you want some real grunt off the line. But of course going to lower gear ratios will hurt mpg's as well as top speed will be lower.

trelos78 07-20-2012 09:06 AM

Re: rear axle gears
 
Got it!!! Thanks for the feedack. I understand that higher gears will lower top speed but do you have any idea how much? the consensus on the forums is that 3.70 is the best of both worlds........off the line and top end. I am most likely going to be installing a whipple twin screw in the next couple of months so I dont have to worry about losing on the top end side????

Yadkin 07-20-2012 09:36 AM

Re: rear axle gears
 
As a rule of thumb, your top speed will be reduced by the same ratio of your gear change, for example 120mph x 3.45 / 3.70. Of course that assumes that your speed is governed by engine rpms.

bit50 07-21-2012 10:23 AM

Re: rear axle gears
 
Quote:

Originally Posted by trelos78 (Post 659113)
Got it!!! Thanks for the feedack. I understand that higher gears will lower top speed but do you have any idea how much? the consensus on the forums is that 3.70 is the best of both worlds........off the line and top end. I am most likely going to be installing a whipple twin screw in the next couple of months so I dont have to worry about losing on the top end side????

I would not be concerned at all with loosing top end. The added gearing will probably help you to go faster as it will give you more torque to fight the wind resistance. And besides who cares if you top end drops from 160 to 145ish. How often are you really going to be going that fast?
Don't fear the gear, as someone already mentioned go 3.90 or 4.10's. Your mileage shouldn't be effected that much and it may even get better around town. And with the 6 speed transmission your rpms will still be very low on the highway.

Please update us if you get this done as I'm very curious about doing this someday myself.

tnjeep 07-22-2012 06:24 AM

Re: rear axle gears
 
I am a big fan of 4.10 gear ratios. Have that ratio in my chevy truck with the smaller V8 (4.8) and it would pull of the line harder than my old work truck (chevy with a 5.3 and 3.73 gears). They also both got the same mpg's.

trelos78 07-22-2012 07:45 AM

Re: rear axle gears
 
Thanks for the feedback guys!!!! Shopping around for prices as we speak. It ain't gonna be cheap, I can tell you that much. I'll keep you updated.

Yadkin 07-22-2012 12:54 PM

Re: rear axle gears
 
The only car I ever drove with 4.10's was my grandfather's '72 Olds. That car would destroy anything off the line, but it topped out at 85. :)

kkreit01 07-23-2012 12:02 PM

Re: rear axle gears
 
I've had a Explorer Sport SOHC with 4:10s, and a Dakota Sport 318 with 3:92s. Both were much more thrilling off the line vs. my WK2 with super-high highway gears. At least the V6s should get lower gears vs. the Hemi (to help make up for hp/torque difference). The Pentastar would be so much better with a decent set of low gears underneath.

Yadkin 07-23-2012 12:30 PM

Re: rear axle gears
 
People buy the Pentastar over the Hemi to save gas, hence the tall gears. If you want to drag race get the Hemi.

bit50 07-23-2012 02:44 PM

Re: rear axle gears
 
Quote:

Originally Posted by trelos78 (Post 659778)
Thanks for the feedback guys!!!! Shopping around for prices as we speak. It ain't gonna be cheap, I can tell you that much. I'll keep you updated.

Ya, it cost me $1500 to get the 4.88's put in my F150.

ColdCase 07-23-2012 04:25 PM

Re: rear axle gears
 
Quote:

Originally Posted by kkreit01 (Post 660250)
.. Dakota Sport 318 with 3:92s. Both were much more thrilling off the line

My 3.92 dakota sport sounded like it was going to explode on the highway at 70 mph.. not a very comfortable ride.

The stock rear end ratio for the V8 is 3.47, V6 3.06. I recall 2.73s and 3.13s being common street ratios years ago.

Overall top gear for the V8 is 2.32, 2.54 for the V6. The V6 has a 3.59 ratio first gear where the V8 has a 3.0 .

The STR8 uses the V6 transmission with 3.70 axle ratios.

Wonder if you could plug in the 3.70 STR8 Difs into the the base V8 housings? V6 housings? Quite a bit of difference between 3.47 and 3.70 gears (near 7%), it may be worthwhile.

Another trick is to use the smallest diameter tire you can get away with.

On the other hand, wonder if it would make more economical sense to just sell the base WK2 and buy a STR8 (if you can get one) rather than screw around with updating base? Bigger engine, more power, better gear ratios for stoplight racing... warranty in place.


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