depends on the tyre wear of the others. if there really low yes its best to replace all of them
Since your Sub is AWD, all tires should have similar tread depths.
we usually like to do them in pairs~ stick to the size indicated on the door jamb...your insurance company would expect no less~
/\ Good info above. We've run into this many times at my shop. If the other 3 are very close in tread depth, within around 2 or 3/32 of an inch differential, you'll probably be okay. But more than that, and you start stressing components. Especially being a Subaru which is All-Wheel-Drive. On Full-Time AWD/4WD vehicles, we always recommend replacement of all 4.
the top of lincoln's head with a penny (if we've still got these)~ there are also 'wear bars' that appear like little sausages goin' across the tire~
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