LOS ANGELES — The Impala — GM's best-selling passenger car in North America — has been redesigned for the 2006-model year with an optional V-8 engine, a rear seat that flips and folds down flat to create more trunk space, and better fuel economy, according to the Detroit News. The sedan, along with the redesigned Monte Carlo coupe, is among 29 new models General Motors Corp. plans for the 2006-model year. "We said that we were going to refresh 90 percent of the portfolio over a four-year period of time," said Gary Cowger, president of GM North America, at a press conference at the Los Angeles Auto Show. "And part of that is refreshing all the models we've got in the mid-sedan segment. It's very important to keep these fresh." The new V-8 engine features GM's displacement-on-demand technology that saves fuel by deactivating cylinders when power demands decrease. The base engines include a 3.5L V-6 and a 3.9L V-6. Flip-and-fold rear seats can transform the interior into several configurations, from a covered storage area to a pass-through from the trunk.