[pjmoreland]

I don't think you need to take into account how much the tire is squishing where it contacts the ground. In the end, the entire perimeter of the tire will contact the ground during one revolution, and the fact that it is squishing at one point won't change circumference of the tire. You could run a test where you put a vertical mark low down on the sidewall pointing straight at the ground. Make a corresponding mark on the ground. Then push your truck forward far enough to rotate the wheel one revolution, and then make a second mark on the ground lined up with the mark on your tire. Measure the distance between the two marks. Divide the result by 3.14 (pi) and you will have the diameter of your wheel.

I am not familiar with the types of speedometer gears that go in your transmission. Hopefully someone who is will chime in.