Captain_Toyota said:
Yup, the bigger tires are killing your milage. Bigger tires mean more unsprung weight. More unsprung weight means that your engine has to work much harder to turn the tires.
you meant rotating mass.

i'm sure you already know, but for others' benefit, unsprung weight has nothing to do with acceleration/braking. it affects handling, ride quality, and suspension responsiveness.
less rotating mass offers a smaller moment of intertia, meaning it's easier to change rotational velocity. more rotating mass has the opposite effect.
wheels and tires are in both categories, hence the huge desire for lightweight wheels for sports cars and road racing. heavier wheels/tires means more rotating mass (bad for fuel economy) and poorer ride quality (bad for sissy men

).
-Mike