back pressure tube

[Brucester1]

had a rusted out back pressure tube. replaced it and the truck runs much better. BUT my MPG went from 15.5 to 13.5 and my air bag light flashes on and off when i start it. was told its a code?

[Brucester1]

'99.5 F250, 7.3. 139000 original miles. when I first got it last year I was getting 13.5 mpg. I put a 4" exhaust and DIY intake and got mileage up to 15.5. Fixed the exhaust back pressure tube and mileage went back down to 13.5. Recently had PCM re-flashed for extra 80HP now mileage is back to 15.5. how do I regain that 2MPG that I lost when I fixed the back pressure tube? Any suggestions would be appreciated. Thank you!