I run 16" GoWesty alloys and correct tires.  But I keep a 14" Vanagon steel wheel (with 5 original nuts and 5 original bolts) with correct rated tire in the clamshell.  Brand new....well...it was 9 years ago.  And all that blowby heat from radiator; no rubber wear, but many cycles of hot and cold, expanding and then shrinking back, a tough environment.  No sunshine of course; ozone exposure same as other tires I guess.  I always keep it properly inflated.

So the question:  time to replace this unused tire?  For me, the spare is only to get to tire store to replace the 16" tire after failure.

Rich
San Diego