From what I remember thats done through the ABS module. Because all rear drive trucks and Vans have at least RWAL on that year with the sensor in the rear diff for rear wheels ABS opperation and speedo opperation also. The RWAL is rear anti-lock brakes as all trucks of that year have at least rear anti-lock brakes and some have all four wheel anti-lock brakes. You can set it as was said with the Mopar DRB scanner as I remember it had a bunch of different tire sizes and different rear ends you had to program in it so it knows the gear ratio and tire size. I also believe if you have a tire size thats not an option in the scanner you can get the "revs per mile" for the size tire you have and set it by that. I used to have a chart they gave us on revs per mile by tire size but I believe most tire stores can tell you that also. Ron