Since buying my Civic, I've only had five tanks that averaged less than 40mpg. Four have been in the last few months, and I'm not quite sure why.
I'm getting no engine error codes. I have noticed that the engine idle varies a bit, sometimes it's at it's normal 600rpm, other times it ranges up to about 1200rpm but otherwise the car drives normally. It may be a little down on power, but with a light foot and low rpms, it's hard to tell.
Most, if not all, of the decrease is during city driving and stop-n-go. This morning, the freeway was pretty clear and I managed to get better than I ever have before on this stretch, 65.8mpg:
I suspect part of this is due to the low rolling resistance tires that I put on in November.
I can't claim the decrease is due to cold weather, since we have had an inordinately warm winter.
Also, last summer I had the head rebuilt, so there shouldn't be any problems with valves, headgasket, EGR, etc. I recently double checked the valve lash and it was within specs.
Any advice on what to check would be appreciated. I have a suspicion, but want to get other opinions before I buy expensive parts...