oxygenated fuel. winter fuel sucks over here too. same thing, winter=bad mileage, summer it goes back up.
my ford did that but i dont think my taco does. hmmm.....Could also be that's the season where you might be using your defrost all the time, which cranks on your AC (automatically) to dry out the air before it hits your windshield. AC on = Bad mileage. Maybe in combo with the bad gas you guys get, too