George-
In the course of transporting a half-ton of CDs plus tents, tables and display cases dor CAMSCO to various festivals, I've used both roof racks and trailers. On a 1991 Volvo, fuel economy dropped from 16 to 22 mpg with a roof box (ca 24 in. tall, 5 ft. wide and 6 ft. long. With a 1999 Subaru Wahon and the same box, mpg dropped crom about 32 to abou 30; with a 5' x 8ft. x 6 ft. box trailer, mileage dropped to about 21 mpg.