An anonymous reader writes: The Toyota Prius somewhat has become the posterchild for fuel efficient cars, at least in the U.S. But it is an open secret that the car may not be quite as fuel efficient as you would expect and if you aren't trying to sqeezed out every possible mile like hypermilers. If you are unhappy with the mileage per gallon performance of your Prius, then there is an actual way to push the mpg rating into insane regions, without having to use slipstreaming on your commutes: TG Daily has a feature (with video) on the car and A123 systems (the guys providing the batteries for the Killacycle), which offers an aftermarket battery kit that apparently can result in 150 mpg or more — or virtually infinite mpg, if you recharge the car every 50 miles. Cost? $10,000.