I have 245/70/19.5's on my dually. I looked up all the info on the tires on the net. Under tire calibtation, I can not get these to match the info from the net. If I enter the tire diameter then the circumference doesnt match, same as revolutions per mile. Thats according to the chart on the net. Whats the better of the three numbers to enter. I entered in diameter and so far the speedometer seems alot closer, wont know to I find my gps.