Tire size can do cause different readings in MPG but will not cause a drastic difference "over night".
A larger diameter tire: 1 rotation = further distance traveled. So you will have actually traveled further than the OD reads. 0.5" difference not sure how much... their was a chart available that shows the MPH differences vs tire size. Larger tire may provide better freeway MPG, could have a slightly lower RPM to maintain a specified speed. City driving will cause higher MPG due to starting and stopping a tire with a larger rotational mass.
A smaller tire: 1 rotation = Less Distance. So you will have actually traveled less than the OD reads. Your actual MPG will be less, also freeway you will need to maintain a higher RPM to maintain the same speed.
Have you checked to see if you might have a dragging caliper or partially stuck drum? if you hit a pot hole and somehow had a huge change in your alignment that could be another possibility.... try testing the DPFE its easy to test.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)