I input 'custom' wheel rim and tire dimensions, and saved that particular rim and tire combination into my dropdown menu as a 'standard'.
I then realized I had the entered the wrong overall tire diameter, so I changed it and saved this as another 'standard' set, with a different name; I would have rather edited or overwrote the initial setting(s) I saved.
I'm guessing there may be at least one way to do this, involving pulling up personal configuration data and editing it...
Is there an easier way (any way at all)?