IIRC from my tanker days in the '70s we ccould have 50% of the rubber off a road wheel before it was considered unservicable. that would be condsidered ESC Red (readiness criteria Red, Yellow, Green). i recall my CSC company commander getting annoyed when i left one of my mortar tracks behind in an alert because of this. (i was also the company motor officer). he asked if it was driveable. i said yes. he said you made your point. go get it.
BTW on M60A1s,A2s (my tank), A3s the roadwheels are aluminum so no rust. the insides have a steel wear plate to rub against the center guides and that will get a patina on it after sitting a week. 10 km road march cleans that right up.