The XXX is the width in mm, the 50 or 55 is the profile, but it's not a straight sidewall measurement, it's the sidewall expressed as a percentage of tread width. This is why a Miata with 195/50R15s actually has a full 1/2 inch less sidewall than a supercar with 315/35R18s.

Must read:

http://www.tirerack.com/tires/tiretech/general/size.jsp

and a must-bookmark:

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

-Philip Maynard '95 Contour [71 STS | Track Whore] '97 Miata [71 ES | Boulevard Pimp] 2006 autocross results