if you're driving ther car normal, you shouldn't see a decrease in MPG.
i was getting 26mpg stock, then went to JB4 and still got 26mpg.
now i'm Cobb Stage 1 and will admit i get @ 23-24mpg ONLY BECAUSE the boost comes on a bit quicker and i like feeling it.
when the feeling of the "new tune" goes away, i'll be back at 26mpg.
on the highway nothing changed from stock, to JB4 to Cobb.