It fits into a wheel hub and can double a car's fuel economy. That's the claim of Dr. Charles Perry, who says his plug-in hybrid retrofit kit can save America 120 million gallons of fuel per day. Big talk. But then, inventors betting on revolutionary uphevals need to talk as big as they think. The former IBM electrical engineer designed the kit to transform existing automobiles into hybrids by placing an electric motor inside each wheel. Perry recently took first prize for his invention at a green energy competition at the Tennessee Technology Development Corp. The plan is to develop the kit into a product selling for between $3,000 and $5,000.