Brad,
Remember that you have to accelerate the wheels to rotate and your present design is pretty massive and has a high polar moment of inertia which is what you have to accelerate. I think you "tread" design is pretty good as you have enough crown and side radius to help prevent to much salt "ploughying" when the wheels are turned but the rim thickness, the "web" between the rim and the bearing carrier and the area that the bearings are carried in are massive and certainly much more than is required. Any of the alloys that you have listed on your drawing will be more that strong enough for a wheel of this size, although the two 7500 series will probably cost you over double the cost of 6061-T6. I did a solid wheel design for a 21 inch dia wheel that would weight less than 1/2 of your present design and have it signed off by the SCTA tech inspectors.
On the other hand if you do happen to crash with these wheels you will probably be able to run them on your next car as they look indestructible!
Rex
