I see by reading the owners service interval manual that the Scion Xa and Xb have been designed for spark plug replacement every 30,000 miles. The first replacement I see for the tC that is listed is at 120,000 miles...at least that is the way I read it. that is the only place that I do not see replacement listed for just "Xa and Xb". I may be missing something or may be blind. Are those iridium plugs supposed to last that long? Not that I will be getting there anytime soon, but I like to know what all the intervals are for my car.