Heavy lobbying by the auto industry thwarted efforts in the Senate to toughen fuel-economy standards for automobiles, particularly light trucks and sport utility vehicles. By a 55-40 vote, the Senate voted yesterday to maintain a five-year freeze on studying possible changes to the corporate average fuel economy (CAFE) standards, which have not been changed since 1975 and which call for light trucks to average merely 20.7 miles per gallon. Sen. Slade Gorton (R-Wash.) had joined enviros and a number of other legislators in calling for higher CAFE standards. The House voted in June to extend the freeze.