PCM and tire size

[MountainManJoe]

The PCM contains data about the size of your tires. It lets your vehicle to display the correct speed. The parameter could also be used by ABS control and possibly affect transmission shifting behaviour. I have no idea if it also indirectly affects engine performance, but it's not impossible to imagine. I know that, for example, engine speed and load is factored into ignition timing.

I went to the dealer to see if they could recalibrate the PCM, but they are only able to input pre-set factory tire sizes (which mine are not). I would like to know in what way tire size (weight is a whole other consideration) changes driveability, and if there are ways to adjust PCM compensation.

[slim724]

what about this from jet performance? The description sounds like it's what your looking for, but at a price of $189.95

LaGrasta here on the forum pointed me to this unit when I was doing my own research on driveablity after a lift and larger tires. Summit also sells them on eBay at $149

SCU 2000
Part #: 50100

The JET SCU-2000 (Speed Control Unit) modifies the vehicle speed sensor output readings to correct your speedometer and odometer after making gear ratio or tire size changes. Shift points in automatic transmissions are also corrected. As a bonus the SCU-2000 also removes the top speed limiter! The unit installs with simple plug-in connectors to the transmission output speed connector and is easily calibrated by adjusting the dials.
*The SCU-2000 is only available for automatic transmissions.

[MountainManJoe]

Sounds like their approach is to fool the PCM rather than reprogram it.
I wouldn't spend $150 on it though.

[slim724]

Exactly, but there's also a convenience factor involved, since it appears to a plug and play item, also with it being adjustable, you could also revert back to stock settings say if you had different tire sizes for summer/winter. And if you ever sold the van you can just remove it and sell it.

How much does it cost to reprogram the PCM?

[MountainManJoe]

How much does it cost to reprogram the PCM?

I have no idea. I haven't been able to find anyone that can do it, or even if it is possible.

[rlsllc]

http://www.lt1swap.com/programming.htm

$75, includes return shipping. I'm having him do the PCM in my 99 AWD to remove the rear O2 sensors and "other" things needed when I put in the new motor.