After jeering at its competitors Ford and General Motors last year for promising to improve the fuel efficiency of their light trucks and SUVs by almost 25 percent over five years, DaimlerChrysler on Friday did an about-face and said it would manufacture light trucks that burn fuel at least as efficiently as those of its rivals. If the automakers follow through on their promises, the changes will affect 7 million vehicles a year, two-fifths of the American market.