A belt that chirps on start-up, glazes on the ribs or walks off the edge of a pulley is telling you which of the three is at fault.
Belt length and rib count are vehicle-specific, and a belt that is one rib off will run for a while before it fails. Tensioners have a service life of their own and are the usual reason a new belt starts squealing within a month. Idler pulleys wear at the bearing first, which produces a rumble that disappears when the belt is removed.
Listings state the rib count, the effective length and whether the tensioner is automatic or manual. Where the manufacturer specifies a stretch-fit belt without a tensioner, that is noted, because installing one requires the correct tool.
