Gap between the front three is pretty much 100km from each other. A couple more seem to have dropped out, bringing the total to 22.
As a complete aside, you can do the whole journey on the train for a rather princely 1200quid one way. Four days, three nights. I've always fancied one of railways great journeys but I'll have to save the pennies for that one.
http://www.greatsouthernrail.com.au/trains/the-indian-pacific