Just a couple things to think about, & try to rule them out...
It's about 10%increase in fuel usage that you have to account for.

A change in formulation of fuel could be responsible for a couple percent. You'll see something like that when they switch to winter formula. It may be that (wherever you live?) the EPA has suddenly required oxygenated gas because of ozone measurements or something like that. That will cause your mileage to drop.

Do you always use the same gas station? How "honest" are the pumps?

What kind of tires? Old vs. new? I probably won'tbe able to help after youanswer, but some tires have more rolling resistance even when they're the same size.

Any reason to suspect your suspension alignment suddenly went out?Some settings can influence drag without causing the car to pull to one side.

 

Maybe the new tires have more rolling resistance?

 

I glanced at my checkbook log and all the gas stations where all different expect for one. So out of 6 gas fillups, 4 were from different places and 2 were the same. I can honestly say that I used to get 35-36 mpg when I drove slower on the highway(60-65 mph as opposed to 70-75 mph).Most of my driving includes 75% highway and 25% city.
Also, my allignment seems to be fine. Like I said before, the car drives as fine as it ever has. The gas mileage has just gone down for some reason and it happened right after I got the new tires.

 

What kind of tires did you have and what kind of tires did you get? Did you replace all 4?
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How long since you changed your spark plugs?