CREATE TABLE table_23840623_4 (dirty_electric_grid_rocky_mountains__denver_ VARCHAR(255), clean_electric_grid_california__san_francisco_ VARCHAR(255))
What is the dirty electric grid rocky mountains (denver) for the vehicle with the clean electric grid california (san francisco) of 160 g/mi (99 g/km)?
    
SQL: SELECT dirty_electric_grid_rocky_mountains__denver_ FROM table_23840623_4 WHERE clean_electric_grid_california__san_francisco_ = "160 g/mi (99 g/km)"