DETROIT (AP) -- Ford Motor Co. said Wednesday it plans to greatly increase the use of more fuel-efficient six-speed automatic transmissions, doubling their number by the end of next year and putting them in 98 percent of its North American vehicles by 2012.

Ford said the six-speed automatic transmissions offer 4 percent to 6 percent better fuel economy than four- and five-speed automatics.

The Dearborn-based automaker says its 6F35 six-speed transmissions will debut in the 2009-model Ford Escape and Mercury Mariner crossover vehicles, which now have four-speed automatic transmissions. They also will go into the 2009 Mazda Tribute and two other vehicles early next year, Ford said.

''These technologies are all about fuel economy,'' said Craig Renneker, Ford's chief engineer for new automatic transmissions.

Ford Vice President Barb Samardzich, head of the company's North American transmission operations, said the new transmissions are key to Ford's planned 30-percent cut in vehicle greenhouse gas emissions by 2020.

''They also deliver improved acceleration and smoother shifting, all at a great value for consumers,'' she said in a statement.

Congress passed a law last year that requires new cars and trucks to meet a fleet average of 35 mpg by 2020, a 40 percent increase from the current average requirement of 25 mpg.

Ford said its Sterling Heights, Michigan, transmission plant will make 1.3 million 6F35 transmissions a year. A $658 million (euro423.75 million) investment announced last year at transmission plants in Sterling Heights and Livonia, Michigan, and Sharonville, Ohio, was primarily for making these transmissions, Ford said.

General Motors Corp. last month unveiled a six-speed automatic transmission combined with a four-cylinder engine in the popular 2008 Chevy Malibu.

GM said the new powertrain would debut immediately in the high-end Malibu LTZ and next year in two lower-priced Malibu models.