The casing is the asset
A commercial tire is a casing that carries tread. When the tread wears out, a sound casing can be retreaded, often more than once. A tire that cannot be retreaded is a tire the fleet buys twice.
What kills a casing
Running under-inflated, overloading, kerb impacts and running on a puncture. Most casing failures are traceable to a pressure problem that was not caught in time.
Removal timing
Removing a tire for retreading at the right tread depth, before the casing is damaged, is a decision worth planning. Regrooving on tires marked regroovable adds mileage before that point.
Cost per kilometre
Casing value at removal belongs in the cost per kilometre calculation. Ignoring it makes a budget tire look cheaper than it is. The calculator in these resources includes it.