I've never really seen it go up from Dv's however I've seen it not go down, but thats driving nice. however like said above, if you're buried in it, you are dumping a lot more fuel and your milage will decrease.
There are no stupid questions.........well nevermind, but this isn't one of them.
Like any mod the milage has the potential to go up since there is less restiction. At the same time it might go down because you'll love the extra power. But seing as the DVs don't become a big issue untill you get lots of fuel and high rpms, they will have a minimal effect at low fuel situations like when you check MPG oke:
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