George, see http://en.wikipedia.org/wiki/Ford_Model_T

"The old nomenclature for tire size changed from 30X3 to 21" (rim diameter) X 4.50 (tire width)."

That was for a Model T. OK, I missed it by 1.5", but on aa rotating tire, installed on a moving vehicle (assuming that was a Model T), over 500 yards, shooting at a depressed elevation, does 1.5 " really make a difference? Somehow this reminds me of Dealy Plaze, but at a much longer distance.