i have a 2010 silverado and i purchased a corsa exhaust system for it, had it installed and now the fuel mileage went down yes went down! the truck is new, has 4,000 miles on and i was getting 500 miles to the tank now down to 440. the exhaust is the sport tuned version and i just dont get it. i was told the mileage would get better because i drive slow no speeding,no hot rodding in this truck kid transporter. can anyone help me does it need to be tuned for the bigger exhaust? please help spent alot of money on this system.