The Duff brothers own a tidy little business empire in the South full of unglamorous businesses like Frozen Food Express, a trucking service with a fleet of temperature-controlled tractor-trailers. Altogether Jim, 57, and Tom, 61, own and run 18 companies with a total of 10,000 employees. It’s enough to make each of them worth an estimated $1.2 billion apiece. (Neither brother would comment on their net worth). The family heirloom, and most valuable business, is unquestionably $1 billion (sales) Southern Tire Mart. Billed as the country’s largest dealer of commercial tires, it sells about 2 million truck tires annually and retreads another 1.5 million tires for customers that include UPS, Penske, and Ryder.