Ken...
Pic 1:
I used a similar shape to your tread segment and copied it around a circle (smallest). Joined them all with a smooth curve.
Then offset the circle to outside the treads (largest), grouped them for a VCarve toolpath
Pic 2:
I then took a copy of the tread pattern and the middle circle, welded keeping outermost
Pic 3:
Did a profile outside the welded vector and vcarved the joined