(FORTUNE Magazine) – Don't buy a Japanese car just to save on gas. As Detroit is boasting in its ads, some 1991 American autos get better mileage than competitors made by Japan. The table below shows comparisons of cars of similar tonnage, number of cylinders, and size. The mileages are weighted averages for city and highway driving. Says EPA project manager John German: ''The myth that Japanese cars do better gaswise is only around because the Japanese made a much higher percentage of smaller cars.'' But Japanese cars have been putting on the pounds over the past 12 years, while U.S. makes became lighter over the same period. The EPA weighed both Japanese and Korean models and found an average 17.4% increase to 2,562 pounds. In the same period, the U.S. average dropped 13% to 3,071 pounds. A Chevy Lumina weighs 3,239 pounds; a Nissan Maxima, 3,086 pounds.

CHART: NOT AVAILABLE CREDIT: SOURCE: EPA CAPTION: MILEAGE SCORES