Chrysler will become the first automaker to offer a fuel-saving 8-speed automatic gearbox with a V6 engine when it introduces its 2012 Charger and Chrysler 300. Highway mileage will be increased 4 miles per gallon, boosting the Dodge sedan to 31 mpg on the highway. A Chrysler spokesman said the automaker is working on a 9-speed automatic gearbox that is expected to be even more efficient. The new transmission will be offered on other models over the next few years.