You must have the patience of a saint to get 36-odd MPG out of a Miata! I thought I was doing fairly well getting over 30mpg from my 1991 car, considering it tends to be driven around town most of the time. The most I've ever got from it was just over 35mpg, and your averaging more than that!
That said, mine needs a service and I might see general improvements with new plugs, filters and some fully synthetic oil. It's probably not in the best of health though it's certainly not bad.
I do enjoy the good economy I can get from it when I'm driving normally but at the end of the day it also gets driven in the manner for which it was designed from time to time. I'm afraid I don't have the self control to manage a full tank without a
little hooning around
Partly why I'm considering getting a more frugal daily driver to go with it, so that the Miata can be used exclusively for fun and the DD exclusively for economy...