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Discussion Starter · #1 ·
I picked my jeep up lastnight with the 4.88 gears installed up from stock 3.73's, no computer programming done...speedometer is dead on. I have a gps app on my phone which tells accurate gps speed. It was dead nuts on before the gear install, and is still right on point afterwards. Are these ecm's automatically adjusting and I/we dont know about it?
 

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Discussion Starter · #2 ·
PS...4.88 gears on an auto with 33's is incredible...feels like a new motor in there!
 

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i think the speed is measured from the actual tire rotation. So your RPM makes no difference (most vehicles have the speed sensor on the trans).
 

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I beleive that speed is measured by the tone rings in your wheels on a JK.
So the gear change would do nothing, other that raise your rpm and give you more power. Someone will correct me if I am wrong.
 

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I picked my jeep up lastnight with the 4.88 gears installed up from stock 3.73's, no computer programming done...speedometer is dead on. I have a gps app on my phone which tells accurate gps speed. It was dead nuts on before the gear install, and is still right on point afterwards. Are these ecm's automatically adjusting and I/we dont know about it?
Not sure who did it for you but they had to have made the computer adjustment for the gear change. My understanding and the word from my builder is that a gear change without the immediate change in the computer will put the tranny almost immediately into limp mode.

Eric
 

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^^^ I think the tone rings at the wheels are used for ABS signal, not sure if they are used for actual speed.
But; when I did my Transfer Case swap; there was NO speedo connection at the output on the TC. As far as that goes, there was NO connection of any kind on the output shaft.
 

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Gear changes don't affect the speedometer. I don't know how it measures speed, but whatever it is isn't dependent on RPMs.

From 2012 on, a programmer is essential as the autos will go to limp mode immediately. Prior to 2012 jeeps, this wasn't the case, so I assume the OP has a pre-2012.
 

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Your speedo is taken from the wheel sensors, and your RPM's are effected by gear changes. That is exactly what your changing in a gear swap. The amount of rotation it takes the axle (tire) to rotate in relation to the pinmon gear (Driveshaft). The JK is pre programmed to run with tires up to 33's without throwing the calibration out. Put 35's or larger on and it will change quickly.

Hope it helps.
 

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MTH said:
Gear changes don't affect the speedometer. I don't know how it measures speed, but whatever it is isn't dependent on RPMs.

From 2012 on, a programmer is essential as the autos will go to limp mode immediately. Prior to 2012 jeeps, this wasn't the case, so I assume the OP has a pre-2012.
Good catch. I didn't think about pre 2012
 

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Discussion Starter · #12 ·
Great info thank you all!!! I was floored when I drove away and the speedo was exactly accurate. I did purchase a Superchips programmer not realizing that it didnt work for the 2011 and up...so Im happy to know that I dont need it for the speedo calibration anyways.
 
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