Automobiles are not singular objects, but rather of a collection of interconnected parts slowly wearing out at different rates. A good mechanic will help identify what needs to be replaced or repaired. According to Reddit user dnalloheoj, the folks at his local Firestone aren't so good at that.

This is only the first act. In the second act he goes across the street to get what he feels is a more honest estimate. See the full graphic here.

(Click on the expand button on the bottom right of the image above to see it full-sized.)