Ever since I had a Magnaflow cat back installed on my Truck I've noticed a significant decrease in gas mileage. Techs @ flow said that this was normal untill ECU relearned free flowing exhaust pattern after driving 300 to 1000 miles, or after a reset of the ECU by removing negative wire frim batt and then allowing to idle then drive 1 complete cycle. haven driven 1000 miles plus 3 resets I have not gained any increase in gas consumption. Have any one else suffered this or similar problems with a cat back system? The exhaust was proffesionally installed and the truck only have 47.000 on the dash.