Table schema:
CREATE TABLE table_name_56 (effic INTEGER, avg_g VARCHAR, gp_gs VARCHAR)

Question: Which Effic is the highest one that has an Avg/G smaller than 305.6, and a GP-GS of 13-13?
SQL: SELECT MAX(effic) FROM table_name_56 WHERE avg_g < 305.6 AND gp_gs = "13-13"