General Motors Corp. says it will put at least 2 million vehicles on the road by 2008 with new engines that incorporate technology to improve fuel economy by as much as 8 percent. A computer in the engines senses how much power a vehicle requires and seamlessly disables or engages half or all of the cylinders without any action from the driver. This process is known as displacement on demand. Beginning in model year 2005, GM's new Gen IV Vortec 5300 8-cylinder engine will be available with displacement on demand in the Chevrolet TrailBlazer EXT, GMC Envoy XUV and GMC Envoy XL — all extended length sport utility vehicles. The Vortec 5300 without displacement on demand will be available in the Buick Rainier, a mid-size SUV. The new 3900 V-6 engine's first application will be without displacement on demand capability when it is installed in the Pontiac G6 which will replace the Grand Am next year. A displacement on demand version is planned, but GM would not reveal when it will be available. "I think it's a foot in the door, a step in the right direction," David Friedman of the Union of Concerned Scientists said of GM's effort. "But 8 percent fuel economy on a vehicle that gets 17 miles per gallon, that's a little over 1 mile per gallon. There's a long way to go." The use of displacement on demand engines could help GM achieve new federally-mandated fuel economy standards for light trucks, which call for a 1.5 mile per gallon improvement, from 20.7 mpg to 22.2 mpg, by 2007, Friedman said.