I measured the tread depth of the old tires, and I measured 0.200 inches - right around 3/16 of an inch.
The new tires are Carlisle Multi-Trac, and the depth is approximately 0.430 - about 7/16 of an inch. That is a full 1/4 inch more! Pretty amazing.
And they look real good mounted on the 420. I expect much better traction, so I kept my wheel weights off. That, and all my mole hills are pretty well mashed down anyways.