Table schema:
CREATE TABLE table_16105186_2 (us_national_average_electric_mix VARCHAR, midwest__des_moines_ VARCHAR)

Question: What was the US national average electric mix rating for the vehicle that was rated 280 g/mi (174 g/km) in the Midwest?
SQL: SELECT us_national_average_electric_mix FROM table_16105186_2 WHERE midwest__des_moines_ = "280 g/mi (174 g/km)"