My 2008 Sport automatic has 103k miles. Last month I was averaging 35/36 (which I always get in cold weather). One weekend I got stuck in 8-10 inches of unplowed snow and did a good amount of wheel spinning, then got gravel stuck behind the left front brake shield (it popped out after several miles when I was finally able to safely pull off the highway). The next day I put on new Goodyear GT's. After that, my milage dropped instantly to 32/33 MPG.

Warmer weather now, I can only get 33/34. I should be getting 37/38.

I don't think it is the tires,This is the same time of year I put on new GT's 56000 miles ago, milage actually incteased with the new rubber. Installed a new air filter, ran injector cleaner thru the gas tank twice.

Any idea why the sudden decrease?

 

Go do a tune up. air filter, re-gap spark plug, if you haven't adjust the valves yet, do it. After all those, reset the ECU. Should help a little bit with the MPG.

 

I tend to get crappier mileage during the winter / spring months than the summer. I always read things about "winter gas" attributing to poor mileage. Try the above listed if you haven't already, but maybe it will start to even out as the weather gets a tad bit warmer and typical gas comes in.