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Old 11-28-2012, 10:48 AM   #1
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Gears

I went from stock Goodyear 225/75 16 to bf Goodrich 255/75 17. What do I need to do to the gears and speedometer? And what is the typical cost for something like this?
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Old 11-28-2012, 11:34 AM   #2
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Hi
What gears do you have now and what trans. Also what year and model. I dont see a spec sheet for your jeep.
Bob

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Old 11-28-2012, 11:37 AM   #3
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Originally Posted by Gunner
Hi
What gears do you have now and what trans. Also what year and model. I dont see a spec sheet for your jeep.
Bob
I don't know the gears now. All stock. 2012 sport
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Old 11-28-2012, 11:40 AM   #4
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you don't have to do anything. if and when you go to 35's and up is when you need to think about regearing. your sport has 3.21 axle gear ratio. so you are good to go. I did the same "upgrade" and don't have to regear and I have 2010.
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Old 11-28-2012, 11:42 AM   #5
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Originally Posted by Havok
you don't have to do anything. if and when you go to 35's and up is when you need to think about regearing. your sport has 3.21 axle gear ratio. but you have a pentastar 3.6 so you are good to go. I did the same "upgrade" and don't have to regear.
Thanks. I know my speedo is off and I'm getting really poor mpg. (15-16). Hoping to do better with the pentastar
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Old 11-28-2012, 11:42 AM   #6
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Most likely 3.21. If you have the window sticker still it will list all the options and gear ratio is one of them. Also stick or auto.
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Old 11-28-2012, 11:44 AM   #7
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Also your dealer can reflash your computer to correct your speedo.
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Old 11-28-2012, 11:46 AM   #8
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Most likely 3.73 gears in the '12 Sport which will be fine up to 35" wheels. Some '12 have 3.21. You would need to check your build sheet. If it's a 4 door and/or has the max tow package, you probably have 3.73.

You can use an AEV Procal to fix the speedometer. It costs around $150. Gearing will not be needed.
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Old 11-28-2012, 11:47 AM   #9
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Quote:
Originally Posted by Gunner
Most likely 3.21. If you have the window sticker still it will list all the options and gear ratio is one of them. Also stick or auto.
Sticker says 3.21. And it's auto
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Old 11-28-2012, 11:59 AM   #10
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Sticker says 3.21. And it's auto
Even with 3.21, you won't need gears unless you install 34"+ wheels.
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Old 11-28-2012, 12:02 PM   #11
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Ok You went from a 29 to a 32 inch tall tire. That means that at 60 mph your really doing 66 mph. That also means that your odometer is 10% slow. So when you show 200 miles you need to add 10% and its really 220. That is going to boost your gas mileage numbers. To optimize the combo you would have to regear to a 3.73. Which is expensive. I would wait to do that until you go up more in tire size.
Bob
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Old 11-28-2012, 12:05 PM   #12
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Quote:
Originally Posted by suicideking

Even with 3.21, you won't need gears unless you install 34"+ wheels.
Thanks a lot. I'm sure ill have many questions for you guys.
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Old 11-28-2012, 12:05 PM   #13
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Quote:
Originally Posted by Gunner
Ok You went from a 29 to a 32 inch tall tire. That means that at 60 mph your really doing 66 mph. That also means that your odometer is 10% slow. So when you show 200 miles you need to add 10% and its really 220. That is going to boost your gas mileage numbers. To optimize the combo you would have to regear to a 3.73. Which is expensive. I would wait to do that until you go up more in tire size.
Bob
Thanks Bob.
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Old 11-28-2012, 12:24 PM   #14
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Just get an AEV Procal to fix the spedo. It's only $150.
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Old 11-28-2012, 12:25 PM   #15
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Quote:
Originally Posted by suicideking
Just get an AEV Procal to fix the spedo. It's only $150.
Perfect
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Old 11-28-2012, 01:40 PM   #16
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Just get an AEV Procal to fix the spedo. It's only $150.
Or save the $150....do the math in ur head....it'll keep the mileage down 10%...and stay in ur warranty 10% longer...
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Old 11-28-2012, 03:49 PM   #17
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Quote:
Originally Posted by RLSmith
I went from stock Goodyear 225/75 16 to bf Goodrich 255/75 17. What do I need to do to the gears and speedometer? And what is the typical cost for something like this?
You basically went from 29 to 32.
My 2012 JK had the 24s package stock, 3.21 with 255/75-17. I added front bumper with winch, 4" lift, and 35 tires. I ran with 3.21 for a while, even off-roading some tough trails and performed fine. I did use the pro-cal due to the 35 tires.
However I since did regear to 4.88 for a better crawl ratio. It is great Offroading and on pavement. The JK will run at a higher rpm with this combo.
In any case do not count on better mpg. I only get around 13mpg on average.
You should run with it as is and see how it feels. Once you start doing some of the bigger mods you can start thinking about some of the more technical issues.
I hope this helps.

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