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Old 06-24-2010, 04:58 AM   #1
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Regearing/Speedo/Odometer

I recently regeared my 08 JK from 3.21 gears to 4.88's and thought my speedo would be way off. Was kinda surprised when I drove it and the speedo looks like it's working fine (I haven't used a GPS yet but I'd expect it would be very noticeable). Does anyone know of a reason why I'd need to get a programmer to adjust the computer to the 4.88 ratio?

I have the manual 6 speed so needing to adjust the computer for shift points like the auto isn't a problem. Was wondering if I'm maybe missing something. Read several posts online that said I'd need to program for the new 4.88 gears.

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Old 06-24-2010, 07:24 AM   #2
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Do you have larger tires as well? Maybe the two changes had opposite effects. I would verify the accuracy against a GPS and go from there if it were mine. How do you like your new gears?

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Old 06-24-2010, 07:41 AM   #3
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Yes, you will have to reprogram the computer to take into account your gears and tires.
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Old 06-24-2010, 07:52 AM   #4
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Question Re-program?

I have an '03 Saraha...I put 32" tires on so my speed-o reads two miles an hour slower according to the GPS. Do I have a gear to change or is this a re-program issue?
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Old 06-24-2010, 07:52 AM   #5
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AEV Procal-$149 bucks, does a lot more to !!

I flasned my JK ECM for diff tire size, TPMS "beeps" and turn signals (3-blink)

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Old 06-24-2010, 08:20 AM   #6
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Quote:
Originally Posted by mistert View Post
I have an '03 Saraha...I put 32" tires on so my speed-o reads two miles an hour slower according to the GPS. Do I have a gear to change or is this a re-program issue?
On TJ's except Rubi's it's a speedo gear, but honestly for 2mph I wouldn't mess with it... Mines 5mph off, no big deal....
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Old 06-24-2010, 02:19 PM   #7
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On TJ's, they use a reluctor wheel, at least the 01 does, and so it can't be changed as it's built into the transmission. It's all controlled by the ECM, so it would need to be reprogrammed.
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Old 06-24-2010, 03:02 PM   #8
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No, you do not need to recalibrate the JK if you change gears.

Only if you change tire size.
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Old 06-24-2010, 07:40 PM   #9
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Quote:
Originally Posted by InfernoGirl View Post
On TJ's except Rubi's it's a speedo gear, but honestly for 2mph I wouldn't mess with it... Mines 5mph off, no big deal....
This might be a big deal if you care about the mileage on the vehicle. Won't that effect it?
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Old 06-25-2010, 08:58 AM   #10
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Quote:
Originally Posted by The Great Escape II View Post
This might be a big deal if you care about the mileage on the vehicle. Won't that effect it?
I agree, it will throw the speedo and odometer off, which could be beneficial in "keeping the miles" off of the vehicle, but would be considered lieing if you sell it like that.

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No, you do not need to recalibrate the JK if you change gears.

Only if you change tire size.
YOU DO have to recalibrate if you change gears. The speed sensor is located on the left side of the transmission. Both the input and the output speed sensors are there. It will not know if you regear the rearend and front diff.
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Old 06-25-2010, 09:01 AM   #11
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Originally Posted by deathphoenix99 View Post


YOU DO have to recalibrate if you change gears. The speed sensor is located on the left side of the transmission. Both the input and the output speed sensors are there. It will not know if you regear the rearend and front diff.
We are talking JKs I assume.. So quit spreading wrong information. I have personally tested my speedo with 3 different gearsets on my 32" tires with GPS. The speed sensor is in the left front ABS area at the wheel. Thusly, gear changes alone do NOT affect it. Once you research properly and determine the correct answer you can come back and admit it.

OMG you dont even own a JK.. why am I wasting my time..
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Old 06-25-2010, 09:28 AM   #12
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Speedo

Think you're dead on Plac (but you already knew that ). Guy at the gearshop told me the same thing. The JK's don't need to be flashed/reprogrammed for a gear swap. Due to the speedo sensor location the JK is reading the axle rotation as the JK rolls down the road so regearing doesn't cause an issue with the speedometer because the speedo gear/sensor isn't in the transmission and not affected by the increase in driveshaft revolutions. (Please feel free to add any corrections if I've gotten it wrong.)

If I ever bump the tires up over the stock 32's I'm assuming I'd need to flash/rreprogram for larger diameter or am I'm not thinking straight?

jk'n - The 4.88's are a good fit for me. I live in the Hill Country in Central Texas and I don't have to downshift the 6 speed like a madman anymore while I'm driving around. If you have the auto you can gear even higher due the crazy high overdrive in the auto trans. I can cruise at 65/70mph at 2800-3100rpm but prob wouldn't push the JK much faster for a sustained amount of time due the high rpm. (I can still pass on the highway up to 85mph but like I said I woudln't hold this speed for a extended amount of time).



This gear chart was dead on for rpm and speed with my 6 speed and the 4.88's. With the auto the 5.38's would probably be ideal with stock 32" tires.
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Old 06-25-2010, 09:39 AM   #13
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Yea, I went from stock 255/75x17 BFG's actually 31.? installed-


Quote:
Originally Posted by Sierra8 View Post
Think you're dead on Plac (but you already knew that ). Guy at the gearshop told me the same thing. The JK's don't need to be flashed/reprogrammed for a gear swap. Due to the speedo sensor location the JK is reading the axle rotation as the JK rolls down the road so regearing doesn't cause an issue with the speedometer because the speedo gear/sensor isn't in the transmission and not effected by the increase in driveshaft revolutions. (Please feel free to add any corrections if I've gotten it wrong.)

If I ever bump the tires up over the stock 32's I'm assuming I'd need to flash/rreprogram for larger diameter or am I'm not thinking straight?

jk'n - The 4.88's are a good fit for me. I live in the Hill Country in Central Texas and I don't have to downshift the 6 speed like a madman anymore while I'm driving around. If you have the auto you can gear even higher due the crazy high overdrive in the auto trans. I can cruise at 65/70mph at 2800-3100rpm but prob wouldn't push the JK much faster for a sustained amount of time due the high rpm. (I can still pass on the highway up to 85mph but like I said I woudln't hold this speed for a extended amount of time).

http://www.steveit.com/download/Jeep/gears.jpg

This gear chart was dead on for rpm and speed with my 6 speed and the 4.88's. With the auto the 5.38's would probably be ideal with stock 32" tires.
To Hankook Dynapro's at 33.2" installed and my TPMS(Different wheels)"beeped" and the speedo was off a little over 1%, which isn't much, but I got the AEV Procal module and "fixed" it

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Old 06-25-2010, 09:42 AM   #14
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Quote:
Originally Posted by Sierra8 View Post
Think you're dead on Plac (but you already knew that ). Guy at the gearshop told me the same thing. The JK's don't need to be flashed/reprogrammed for a gear swap. Due to the speedo sensor location the JK is reading the axle rotation as the JK rolls down the road so regearing doesn't cause an issue with the speedometer because the speedo gear/sensor isn't in the transmission and not affected by the increase in driveshaft revolutions. (Please feel free to add any corrections if I've gotten it wrong.)
everything you said is correct.

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