will a bug deflector decrease my gas mileage and if so how much will it decrease

 

I doubt if you will notice any difference in your gas mileage. The truck still has the same frontal area (includes cab), as you aren't changing the height or width of it, just the shape. The bug deflector I had on my old GMC didn't do much to deflect bugs, so I took it off and never missed not having it. It did help some with reducing rock chips on the hood though. With a 454 and 4.10 axle, gas mileage was the same with or without bug deflector.

 

I'd have to agree with SoCal on this, you wont see any difference in your mileage or the lack of bugs on your windshield. If your buying it hoping you wont have to scrape bug carcasses off your windshield, dont bother. They do work to reduce hood chips as said. Another thing you will notice is that if you get the smoke plastic ones, they will get cloudy and scratched within a few years leaving them looking pretty shabby.