When 100 g/mi (62 g/km) is the clean electric grid california (san francisco) how many vehicles are there?
create table table_24620684_2 (vehicle varchar, clean_electric_grid_california__san_francisco_ varchar, PRIMARY KEY (vehicle))
select count(vehicle) from table_24620684_2 where clean_electric_grid_california__san_francisco_ = "100 g/mi (62 g/km)"