255 is width
/??? is "aspect ratio." You could think of as a percentage of the treadwidth to make an estimation. Or you could use a tire size calculator online to find a size that's close to what you want. 255/60-15 is about 1/2" taller tire. 255/70-15 is a bunch taller, about 3". That would lower rpm on the highway by 10%.
http://www.miata.net/garage/tirecalc.html