[DavidG]

Franki,

As the survival rate of metal covers such as those used in '35 and '36 are fairly high, not all of them were left at the side of the road. A lot of them likely remained unused in garages or attics or basements as they were a pain to remove and reinstall, with the likely exception of the '36 cover where only one set of hands is required to reinstall one, not two sets of hands like the others. The irony is that the fabric covers were relatively easy to remove and reinstall, but their material composition was no match for extended outside exposure.

In any event in answer to your question likely a couple of years after the cars were new the majority of their spare tires were no longer sporting their tire covers.