Mercedes sedan ups fuel mileage

Most people don't buy big, heavy, luxury sedans for their gasoline mileage. There are many other vehicles — lower-priced, too — that excel at fuel economy.

But government goals for atmospheric carbon reduction, plus competition among automakers, can do funny things to the car marketplace and help explain why Germany's Mercedes-Benz now sells a gasoline-powered, hybrid version of its flagship S-Class sedan.

The new-for-2010 S400 Hybrid is the first mass-production car to use a lithium ion battery to store and supply electric power. The 120-volt battery is more compact and adds less weight to the car than do nickel metal hydride batteries and has high energy density for its …