Advocates who want to reduce the use of fossil fuels through improved mileage for gasoline and diesel vehicles have a new target—human fat.No, they're not talking some macabre new biofuel. They're simply pointing out that it takes more fuel for big-bellied, broad-butted Bubba to drive around in his full-size pickup truck than for skinny Minnie in her small economy vehicle."One socioeconomic implication of obesity is that it reduces passenger vehicle fuel economy," said Sheldon H. Jacobson, director of the simulation and optimization laboratory in the computer science department at the University of Illinois.In 2006, Jacobson and doctoral student Laura McLay found US cars a...