Bill,Sting rays came out with both s2's and s7's,The s2 is for the back wheel the s7 is for the front wheel,s2's are a wide 20.125 for a wide slick or studded tire, the s7's are a 1-1 3\4 ,it is its own size u cant put any other tire exept a schwinn fit 1-1 3\4,Sting rays had this set up for the racy fat back skinny front,and a visual rake of a race car.Now not a sting rays had this set up,some junior models had both s7's front and back,be sure you dont have a junior,if your bike has the MAG sprocket, it is a s2 bike,if it has a small lucky 7 it could be a junior,63-67 rays now did have s2's with a lucky 7,and large sprocket bikes in 67 are s2' bikes,if you have the back fender if its wide some measure 3 across,and smaller s7'fenders will rub on a s2 with a slick,There is a lot to know about sting rays,im just scratching the surface,and still learning,Good Luck on your Ray! oh yeah i think u need a 28 spoke s2 with a red bannded bendix,hope i was some help.