So if you change your wheel/tire size to something that's measurably different than the stock 225/45R18, is there a way to get the speedometer to read the correct/actual MPH? In other words, I'd like to change the "revolutions per mile". I'm hoping one of the various tuning options at our disposal is capable of this...

 

I too am in the process of changing tire sizes, but all that I really found on the subject was that mazda can apparently change some setting. However they can only select predetermined stock tire sizes.

What is interesting is in MazdaEdit logging I believe I did see tire size, but no changeable/programable tire size option in MazdaEdit.

 

Nope, no way to change it on the vehicle.
honestly, not a big deal, many of us are running different sizes with zero issues.

if you really want to know how much the speedometer will be off, you can utilize this: https://www.miata.net/garage/tirecalc.html