Hi
Go to this site - you can type in the new tire size and it will tell you the difference in your speedo and circumference

http://www.miata.net/garage/tirecalc.html


I checked it for you and when your speedo shows 60mph - you will actually be going 61.9mph - also your circumference is 2.4in taller

If these tires aren't installed yet, I would go buy the proper size for them - a taller sidewall means your cornering won't be as good, and depending on your shocks' quality, you could bottom out when you have lots of passengers or a full trunk

As far as I know, you won't be able to recalibrate the speedo

Take it easy