Gas Mileage is dropping.

[vic7c]

I have 2008 that I have owned since Oct 1 2012. Approaching 105,000 miles. Notice mileage has dropped from 30-35 to about 25 miles per gallon this winter. I've been told this is normal but I don't remember it lowering in previous winters. I have not gotten a tuneup...I have gotten regular oil changes in 16 pt. look ins with the deal I made..Just got new brakes and tires a few months ago. Could the new tires have caused a problem if not aligned right or should I just get a tune up (maybe check fuel injection)...should I go to a Toyota dealer or stick with the place I get free oil changes, which is a large dealer but doesn't regularly work on Toyotas?