I’ve got a 2021 6.2L. I swapped out the stock muffler for a flowmaster super 50 and also got rid of the flapper valve. I also installed the cold air inductions cold air intake. I am getting less than 13 mpg now compared to almost 16 mpg before. Any ideas what the problem could be? I am not the most mechanically inclined when it comes to vehicles. But I was expecting an increase in mpg rather than a decrease.