Toyota Motor Corp. boasts that its hot-selling Prius gasoline-electric hybrid doesn't have to be plugged in. But a growing number of hybrid buffs interested in further boosting the car's fuel economy are asking, "Why not?" By replacing the Prius' batteries with a more powerful array and recharging it using a standard electric outlet at home, engineers have enabled the hybrid to get more than 100 miles per gallon of gasoline.... But the plug-in Prius is the firm's principal claim to fame. In May, Energy Control Systems entered its Prius in the Tour del Sol fuel economy rally in New York....