Which version of AutoCAD are you using?

You want to cut the wheel in half, so that you're left looking at half of the wheel, i.e. a semi-circle.  Then you want to dimension from the centre, to the circumference but indacate the diameter not the radius?  It sounds a little "dangerous" to me as you'll be indicating a measurement that isn't true but it can be done, easily.  Just use text override and enter whatever value (and text) you want.  I would indicate that it's the diameter as well, just to clarify things.

HTH