I has mine stacked, but this spring I picked up a tire rack- similar to this one:
Tire Rack I think the sale price ends today.
Fortunately I have a 10 foot ceiling in my garage, so I was able to put the rack a little more than 6 feet up in the air to allow being able to walk under it. If you are doing the same measure cerfully- as the tires will hang down a bit as they sit in the rack.