Well after getting 15 mpg for two weeks (i thought the first week was a fluke or something) in a row I checked just one thing and then replaced it. Can you guess what it was? Don't scroll down if you want to.
I should have known
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it was bad? didn't you notice weird engine temps?

odd, I have same rating (180) and I'm doing good on gas winter-wise.

a 180* thermostat shouldn't cut your mileage in half....

Nope it wasn't bad and the temp sat at the 175 mark just like it always had when ever I looked at it since I've had the car; until I looked at it alot more often. Then I began to notice it would drop down (150) and climb back up only to drop way down again. I figure the themostat would open, let in the really cold coolant, close shut again, and repeat.

The new Stant stays right at 200 and boy do I have HEAT now.

That would be mixed city and highway. I took it for a long ride and am already at 21mpg even with most of what I had for miles came from the old T-stat.
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I just got gas so I'll see how this week goes before I start looking at other possibilities.
An O2 is definitely on the list.

In the winter I notice 2-3MPG...no more than that..unless it is stuck open. then your miliage would really suffer.

The Civic drops 6-8mpg in the winter

What I *think* was happening was that i was dropping in and out of open loop and/or that the transmission wasn't able to completely warm up. I noticed my rpms are under 2k on the highway now @ 65