Detroit — General Motors Corp., in an effort to increase gas mileage by an average of 2 miles per gallon, soon will use new "displacement on demand" technology on many of its large trucks and sport utility vehicles. Vortec V8 engines will be installed with the new technology beginning in the 2004 calendar year, spokesman David Roman said Monday. Using computer sensors, the technology automatically shuts off 4 of the 8 cylinders during normal driving conditions. All 8 cylinders are reactivated during acceleration. GM hopes to install about 1.5 million of the fuel-efficient engines by 2007.