Geo Again Tops EPA Mileage List: After playing runner-up for a year, the Geo Metro has regained its status as the stingiest gasoline sipper on the road--at 46 miles per gallon. The dozen most fuel-efficient 1996 model automobiles all recorded combined city and highway driving of at least 35 m.p.g., although most were in the 20 to 29 m.p.g. range, the Environmental Protection Agency reported. The highest-mileage cars are subcompacts, but two Mazda models--the Protege and the 626--lead the compact and mid-size categories. The Lamborghini Diablo again registered the worst mileage, at 9 m.p.g. in the city.