My father is notoriously frugal, and I tell him he should replace his old dry-rotted tires even though they still have plenty of tread. Can you please explain why I'm right?
I think our dads might be brothers. Tire replacement is pretty straightforward to the cost-conscious: when the tread wears down or a hole can't be patched. The reality is more nuanced than that. Tires are complex components that undergo amazing stresses during even the most mundane of drives. It took the better part of 50 years and a world war before motorists could depend on reaching a far-away destination without fixing one or two flats. As rubber technology and steel-belting techniques improved, tires became a very reliable part of the carwe just don't expect blowouts and punctures.
Tires do degrade over time, though, and that process is called dry rot. Oils and chemicals in the rubber compound start to evaporate or break down because of UV exposure. The rubber loses its flexibility and begins to crack at the surface, and the structure becomes more and more brittle (think of a really old rubber band), leading to sidewall damage and eventual failure. And we're not talking "Oh, I'll just fill it up and drive on it"; this is a complete loss of function. You might even see tread start to separate. It's good practice to replace tires as soon as you see signs of dry rot, to prevent blowouts and the subsequent loss of vehicle control. Even if there are no signs of rot, the industry standard is to swap out tires before they hit 10 years old, and some tire companies recommend replacement as early as six years after manufacture.