Great question Tom !
I will check around. We should know this, right ?
If anyone can address this & maybe a source for a new spare tire that is within reason cost wise, I'd be thankful.
My particular spare tire is: Dunlop Space Mizer T155/90 D 18...rating 113M.
It was made in October 2001, per the sidewall date code.
This is on our 2002 ML500.
I noted it leaked down to empty, on vacation last summer in Colorado, after filling it shortly before departure.
Concerned, I observed that the valve stem had rotted thru, so it was bad, not the tire (yet).
A tire shop replaced the valve stem, and I refilled to 60 psi per the spec.
Now, a year later, I just pulled it out for my impending Rear Shock R & I, and it showed 40 psi remaining.
So, this suggests my spare is still OK. But it's 15 years old, and probably time to replace it just to be prepared.
Have any of you replaced your spare, yet ? Details please....
Just an FWIW account....
Cheers, David in East Texas