Fuel economy standards for 1987 and 1988 automobiles will be eased, sparing some auto makers, including General Motors Corp., millions of dollars in penalties for past fuel economy law violations, government sources said. The Transportation Department is expected to announce the change in government fuel economy requirements later this week, possibly today. According to the government sources, the average fuel economy standard for a manufacturer's fleet will be lowered from 27.5 m.p.g. to 26 m.p.g. for 1987 and 1988 model cars.