195 (width) / 60 (profile, how much meat there is from the rim to the top of the tire, lower numbers = low profile) / 15 (the size of your rim)

 

FYI, the 98-02 4cylinders have 195/65-15!!!

 

just to add the sidewall size or aspect ratio is a percentage of the width of the tire. i lot of peeps think that a 60 series sidewall is the same for a 195 and a 205, but it is not. for example 195 * 60% is the sidewall height this is why a 205 55 15 is about the same overall diameter of a 195 60 15.

 

make life simple and just use this

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