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My “outsole test” can tell you a lot, but it won’t help you track the mileage on your current pair.

For many years, I used what I called “the outsole test” to determine when I needed to replace my running shoes. It’s exactly what you’d assume—I’d give the soles of my shoes a quick eyeball and look for areas where the tread had worn away. If the flex grooves had all but disappeared into the surrounding rubber, it was time for a new pair.

I’ve since learned that my outsole test was a pretty bad way to tell if I needed new shoes. Depending on the type of materials used, running shoes won’t always show signs of the deterioration consistent with the amount of mileage you’ve put on them. Your shoes can be shot even if the rubber still looks decent. Conversely, a shabby-looking pair may have a solid hundred-plus miles of running left in them.

That said, you can still learn something from studying the underside of your trainers. You may find clues as to where your foot strikes the ground first when you land. Lots of lost rubber near the heel can indicate you’ve got a strong heel strike. Or, if you’re like me, you might notice that one shoe has more wear than the other. (In my case, this is because I favor my left foot more after a previous surgery.) Very asymmetric wear patterns could signal that you’re utilizing one side of your body more than the other when you run. Of course, you’ll want to rule out your running surface first before you go changing your stride. Crowned roads, for example, can impact the rate of erosion across portions of the outsole.

The best way to know for certain when you need to retire your current pair is to track your shoes’ mileage. There are several ways to do this. You can keep a tally by hand on a page of your training log, but the easiest way is to use an online platform. Many websites and apps will let you add multiple pairs of running shoes. Every time you track a workout, you can select the pair you wore, and your shoes’ mileage will update automatically. You can even set an alert to remind you once a pair passes the 400-mile mark.

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