Four years ago, a sharp spike in gasoline prices helped push the three U.S. auto makers to the brink of collapse. Now, $4-a-gallon gas is having the opposite effect: it is helping to keep U.S. sales on an upward track. Last month, consumers' thirst for small and fuel-efficient cars was a prime factor in driving light car and truck sales nearly 13% higher than a year ago. General Motors Co. said it sold more than 100,000 cars that get 30 or more miles on a gallon, its highest ever and nearly half of the 231,052 vehicles it sold.