Looks like you've already gotten some good responses but I'll chime in since I have an LX and experience some flux in mileage as well. You will certainly need to drive more and get a few thousand miles to really get a sense of the mileage.
Upgrading to the 225 is definitely going to hurt your mileage, a lot more than you think it should. I have about a 4mpg difference between my summer tires, 225 width, and my winter tires 205 width. That said I hit 34-37 with the summer tires on and usually just about 40 with the snow tires.
Wheel and tire weight make a huge impact as well. The XXR while a nice looking and well priced wheel is super heavy. I swear the set we bought was about 30lbs a wheel. They will knock a couple mpgs off also. So just on those 2 factors you're looking at about 5-6mpg which added to your current 26 would put you into the low 30's. I'm almost certain you will see improvement the longer you drive your car and you should start to see mpg in the 30 range.
Definitely do what Monk said and go fill up, get right on the highway, set the cruise control and go for a while, then fill up again and see what you got.
You should be able to get into the mid to high 30's at least. Keep us up to date on how things work out for you and if you mileage starts improving or not. Using the A/C hurts these cars in city driving bad. Highway not so much.