I went from stock sized 2008 SES snowflake wheels with 205/50/16 Yokohama S-Drive's and got about 32-33 mph with pretty passive driving.
Now, no matter how gramps I drive my car with the 6 spoke 2004 SVT wheels and 215/45/17 Kuhmo's I CANNOT get over 30 mpg on the exact same trips.
I'm a bit aggravated and thinking of getting an intake/tune to help with this a bit. :-\
...It's worth it, though. The car looks 100x better on the new wheels.