Calculate the percentage of American cars among all cars.
American car refers to country = 'USA'; percentage = divide(count(ID where country = 'USA'), count(ID)) * 100%
SELECT CAST(SUM(CASE WHEN T3.country = 'USA' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(*) FROM data AS T1 INNER JOIN production AS T2 ON T1.ID = T2.ID INNER JOIN country AS T3 ON T3.origin = T2.country