On the highway? That's disappointing.
I usually get 22ish(60mph) mpg on my stock bridgestones headed to work. This fall our company is moving, so ill be going about 70 then and now at 70 i'll usually sit around 19-20, so losing 1-2 isn't too bad.
What tires did you move to? These falken's are almost 2" wider than my 265/70/17 i currently run.