Johnathan Goodwin walks to the back of his auto conversion shop in Wichita, Kan., and lifts up a gas nozzle connected to a huge cube-shaped container. The orange stuff he's pumping is the key to his company's mission: converting the worst gas-gulping SUVs into cleaner, meaner machines. This is 100 percent canola oil, refined to biodiesel," Goodwin said. His well-maintained shop is a bit like a showroom for that much-maligned symbol of environmental ruin: the Hummer. The silver H-1 - which Goodwin says gets 60 miles per gallon - has already been modified to run on biodiesel, diesel, vegetable oil, gasoline, ethanol, hydrogen, natural gas and propane. On a standard gasoline-to-biodiesel conversion, Goodwin starts by taking a new nine-mile-per-gallon Hummer and removing the original gas engine. In goes an off-the-shelf GM Duramax engine that runs on diesel fuel. A few extra modifications and a tank full of biodiesel later, the Hummer - now boasting 500 horsepower and getting about 20 miles per gallon - is ready for the road. He offers a couple of lower-cost options, including a fuel vaporizer for $1,000 that he says boosts fuel economy by 30 percent, and a $500 software download that reprograms diesel engines to get up to an additional seven miles per gallon. His work has many wondering why the big automakers can't simply reconfigure their assembly lines to make their own cars run as efficiently as Goodwin does. "I don't know why GM hasn't done it," says Goodwin. "But I can tell you that all the parts that I use for the conversion - 95 percent - are all GM parts. I'm not reinventing anything."