3000 miles/tank could be only 150 mpg, which has been done many times. I know an inventor who has run another car on ONLY THE EXHAUST of another car. He did hav to remove the catalytic converter on the source car, of course. Another inventor friend of mine got 70 mpg on a nissan 280zx or sumthing, by doing wut the stupid catalytic converter does, using waste heat and metals to crack the fuel. It's a lot of work and potentially dangerous. Another frind of mine's uncle worked at Ford. They were installing hi-mileage carbs one day. That same day it was discovered before they left the plant, and they were all removed. To top it off, i asked a VP of Sunoco in a public live forum about these technologies. He was a little short on words--just muttered that "yes that happens, no doubt".
How was he recovering the energy from the burning hydrocarbons without cooling the metal catalyst below it's optimum operating temp? Thermoelectric generator ala deep space probes? Some kind of exotic low temperature catalyst?
Exhaust can get to around 1500 F. The metal catalysts only need about 700 F, so there's plenty of heat.