Let's not argue about things that we cannot clearly define. As for viewership, you would have to include the rest of the world and that's impossible to do since there is no Nielsen outside of America. You can't go by the Speedways numbers since they just list everyone with a TV set in the countries that the 500 is broadcast and I think that it's safe to assume that not everyone is watching the race. But I can guarantee that outside of the USA many more people watch Indy than any of NASCAR's shows. Is that enough to overturn the disparity here in the states, who knows?
Please, no one post back that you used to live in Germany or Japan or Belgium and when you went to the bar people either were or were not interested in the 500. Maybe you didn't go to the right bar. Unless you polled every person in every country that argument just doesn't hold water.