If I modify the factory exhaust on my 98 F150 will it Lower my MPG?

 

I added my single input dual output Flowmaster exhaust about 3 months ago and I really didn't notice a huge chance. Maybe at most another mile to a gallon. But then again, I didn't get it for the improved gas mileage, I got it for the sound! As every truck should have a nice rumble to it, to strike fear into those bumble bee cars!...LOL!

 

I put the flowmaster 40 on my '03 screw and my gas milage we down due to me punching it more for the sound....