You can expect seasonal changes to affect your mileage. The summer is bad because of using A/C. The winter is also bad because the Civic won't autostop below 41 degrees (at least, my 03 won't - I'm assuming the 05 runs on the same rules). However, I plan to do a lot more manual shutoffs this winter to compensate. Also, I've found that rain reduces mileage as well (the extra resistance of the water on the road). My best months are Aug-Sept (mild temps so no A/C, and no rain).

*** Retired after 65,000 outstanding miles ***