When was the last time you changed your plugs and wires? I had a similar gas mileage issue earlier this year. Turned out my plugs were sh!t (not to mention 3 Champions and a Boush). I had gotten the car about 5 months prior and hadn't had a chance to (nor did I pay it any mind for some reason) to change them. Needless to say, I was furious and I walked to Kragen for a new set of NGKs. Point is, after I changed them, I got better fuel economy. Also, check your fuel filter, air filter, and fill up with Shell (if you don't already). I've noticed very nice results when I use Shell gas vs others, but that's just my opinion.
Kevin- '90 GSX "Canyon Carver"