use that, punch in the tire that you will eventually get on the 19s and find a tire for the 16s that are about the same size if thats what you want.
dont ask about the miata thing. i dunno
you can run a 235/35-19 in the front & on the outside rear, inside rear you can run a xl 225/60-16, i think thats the same sizes/combo that ekstensive ran on their dually