I put Flowmaster 50 Delta Flow SUV muffler no mine (single in single out as 04 Expeditions have no room to have a second pipe routed). It increased my highway mileage by .5-1.5mpg, almost 2mpg at high speeds like 80mph. City driving, because the larger pipe causes a slight loss of low end torque while increasing high end horsepower, lowered by like .3-.5mpg.