I found the rev's per mile on Tirerack for your 'yokes, and it's 680. According to the FTE article about reprogramming, the new calibration constant should be 9.18.

To reprogram it, you have to ground the wiring connector under the dash, directly under the glovebox. I used a test lead with little aligator clips. While holding the reset button on the odometer, turn the key on, but not running. Press and release the reset button again to display the cal. constant, then use select to change the constant. Press and release reset one more time to lock it in. This can only be done a total of 6 times ever with your current PSOM, so do it right the first time! If you chicken out, you can shut the key off before the final press of "reset."

I found that I didn't get a good ground the first time, and it wouldn't let me change the constant.