Tires
Rotation patterns, wear limits, pressure and alignment.
Tires are the only part of the car that touches the road, and they are also the maintenance item most often neglected until something visible goes wrong. Rotation, pressure and alignment are the three levers that decide whether a set lasts 35,000 miles or 65,000.
Uneven wear is diagnostic information. Both edges worn means chronic underinflation. A worn center strip means overinflation. One inside edge feathered means camber or toe is out, and no amount of rotation fixes that — it needs an alignment.