You didn't state the size of your 15s, but I
suspect 205/55-16 is going to be closer. The
1998 SVT runs this size (they changed it to
215/50-16 to sharpen things up a bit more).
You can compare your tire sizes using a tire
size calculator. There's one on this site you
can use:
http://www.miata.net/garage/ It'll help put your mind at ease about change
in the speedo readout.
T.
