What indicates that tires may be under-inflated?

Excessive wear on the edges of the tires is a clear indication that tires may be under-inflated. When tires are not properly inflated, they tend to flex more than they should, causing the edges to make more contact with the road surface. This can lead to uneven wear patterns, especially on the outer edges of the tread, which is a strong sign of insufficient air pressure. Maintaining the correct tire pressure is crucial not only for tire longevity but also for overall vehicle safety, as under-inflated tires can affect handling and increase stopping distances.

The other options, while they might relate to tire condition, do not directly indicate under-inflation in the same way. A smooth surface might suggest excessive wear but not specifically linked to tire pressure issues, while a lack of visible tread indicates general tire wear, and the air pressure warning light serves as a more direct method of alerting drivers to low tire pressure, rather than being a physical indicator of wear.