i have 2000 4.2L v6 and my fuel mileage has dropped from 15 to 12.5 in the city, but about the same for the higway. This change has happened after installing a flowmaster series 40 muffler. i scanned my truck and found that my tps is at 18% at idle and maxing out 93% with the pedal at full throttle, not at max rpms. anyone has any advice will be welcomed.