If you want to discuss the ride height of your classic Mustang, measure it like this. Doing so eliminates differences in tire size from the equation. This way, only spring height is compared. It allows two different people having the same car to be able to compare the height of their springs, even if they have quite a difference in the diameter of their tires.

The dimension, shown as RH in the picture, is measured from the center of the wheel to the very top of the fender opening or quarter panel wheel opening.