I have been searching the forum to be sure I did not post about something that was already listed some place. But the places I found mentioning belt tension are all for other Scion models as near as I can tell. I read one post where they were replacing two belts. I had to run out and lift the hood because I was sure I only have one. I did only find one. It seems to me that the manufacturer would have a specific instruction on tightening an XD belt. I can remember in the old days before spring loaded belt tensioners on serpentine belts when many people would knock out water pump bearings and generator/alternator bearings from tightening belts too tight. There must be some actual factory instruction that uses some accurately repeatable test or measurement other than how the belt sounds or does it seem about right when pushed on by hand.