for an 18" 235s usually want a 8" rim for 245s its 8.5" rim for 255s its 9" rim for 265 its a 9.5" rim for 275 its 9.5" rim for 285 its 10" rim for 295 its 10" rim for 305 its 10.5" rim for 315 its 11" rim for 335 its 12" rim for 345 its 12-13" rim
These recommended sizes are so the side wall is flat and not buldged out or concaved in.