a change from 30.5 to 29 is not a large or even sudden drop. Winter blends impact it but that should have been used sine November time frame for most gas stations. How do you measure your MPG? did you fill up the exact same station same pump? did you let it auto click off and leave it at that or did you fill more in? if you fill up at a different station, and are a a slightly different incline you can put more or less fuel in. the auto turn off will vary with temperature and with pump. You need to trend it over time. I would not trust a MPG change unless it showed constantly lower for more than 3 repeated fill ups at the same station using the same pump.
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)