Frozen caliper(s) will haul down the fuel mileage noticeably. So will dirty MAF sensor and TB. I had dropped off from 22mpg highway to 19mpg, cleaned the MAF and TB, changed out the plugs just because, air filter was fine. Mileage slowly climbed back up to ~21mpg where it remains now (the dreamometer shows around 23mpg to make me feel better
). Easy to check the calipers if you have an IR thermometer - drive without doing much braking and then measure the temps of the rotors all around - you may find one or two significantly hotter than the others (significant being a couple hundred degrees difference in my case).