Gear change & speedometer adjustment

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Ben There - 22 Apr 2005 20:46 GMT

When I changed rear gears in my old 94 Dakota I had to change a gear
on a sensor in the transmission also but someone told me that you
don't have to do that on the later models. I have an 04 now and am
changing from a 3.55 to a 4.10. Do I have to do anything special like
reprogram the PCM or will it pick up the vehicle speed off of the axle
speed sensor ?

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Tom Lawrence - 22 Apr 2005 23:20 GMT

> When I changed rear gears in my old 94 Dakota I had to change a gear
> on a sensor in the transmission also but someone told me that you
> don't have to do that on the later models. I have an 04 now and am
> changing from a 3.55 to a 4.10. Do I have to do anything special like
> reprogram the PCM or will it pick up the vehicle speed off of the axle
> speed sensor ?

Since vehicle speed is determined by the RPMs of the rear axle, the gear
ratio itself is irrelevant to the speedometer reading. You can swap gears,
and your speedometer will read the same as it did. Changing tire size, on
the other hand, will necessitate a change made in the PCM to correct for the
different revs per mile.

Now - with that said... if you have an automatic transmission, the PCM will
be confused, because the transmission output speed sensor will no longer
agree with the vehicle speed sensor, because the PCM still thinks you have a
3.55 rear. This will cause shifting irregularities with the transmission.
Therefore, you should stop by a dealership and have them re-program the PCM
with the new gear ratio.

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