What was the US national average electric mix rating for the vehicle that was rated 280 g/mi (174 g/km) in the Midwest?
create table table_16105186_2 (us_national_average_electric_mix varchar, midwest__des_moines_ varchar, PRIMARY KEY (us_national_average_electric_mix))
select us_national_average_electric_mix from table_16105186_2 where midwest__des_moines_ = "280 g/mi (174 g/km)"