I found this tool the other day on iTunes of all places and I thought it was very cool. It’s called PICgauge. It’s an app for iOS devices (5.1 or higher) that lets you gauge belt wear on your engine’s serpentine belt without removing it from the car. As the belt wears the gaps get deeper and wider so the ribs get smaller relative to each other. Eventually the belt starts to ride only on the ribs and may start to slip.

The measurement process is very simple: Grab a silver or white Sharpie and draw a line across the ribs of your belt and take a picture of it with your iPhone without using the flash.

Open the picture in the PICgauge app. Tell the app how many ribs your belt has and use two fingers to resize the photo to fit between the lines.

Mash the “measure” button and you’re done. The app finds the boundaries of each rib, measures them relative to each other and tells you if you’re OK, marginal, or kaput. Pretty cool.