A compression check might give you some idea.
Other factors also apply.
Cars generally last longer in some areas than others.
Short trips vs. longer higher average speed trips are another.
Rough roads cause higher wear.
There are a few things I do that seem to prolong life of the power train.
Change oil frequently, and more so for the first 10-20 thousand miles.
Change tranny and rear end lubricants early, initially at about 5k to
determine if there is any metal in the sumps, that at about 12k, when
everything is completely "broken in".
An arguable item is to change the antifreeze, etc. early on, while the
warranty is still in place. In the past, I've had some cars that the OEM
put a stop leak variety into the system when the cars were made. Later
on, I had repeated troubles with their radiators, (multiple repairs)
until I replaced them. One, an 81 Datsun, also ended up with a warped
head that had to be re machined.
A 99 Miata will eventually need a new radiator, if it hasn't already.
The plastic top usually splits at a seam, transition, or mold mark.
You can expect the usual issues with timing belts, water pump, dried out
gaskets and seals, etc.
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