When you have an 8" wide wheel, the measurement is from the tire bead seat that is inside the lip to the other tire bead seat. Measuring from the outside of the wheel lips is not the correct way to measure, so that's why you got 9".
The factory stock size was 225-70-15. Many people use 255-60-15, which is close to the same diameter as original, but wider. There are many other opinions on what sizes to use.