Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: 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?

### Accurate SQL ###

SELECT MAX(effic) FROM table_name_56 WHERE avg_g < 305.6 AND gp_gs = "13-13"