I'm experiencing this drop in my FE.

I attribute it to the (much) colder weather we've been having this week (I'm in central IL) and to the fact that much of my driving is in town, so the car never really gets to warm up completely. At least, not for very long. By my estimation, nothing has changed in regards to driving style/distances/etc. Just the temperature. But it's disheartening to watch the mpg average continue to sink!

So, is a change this dramatic in my FE really the weather, or something else going on?

I drive lots of short trips with a lot of stoplights too. I filled up a tank at 61.5 mpg on Monday night. Cold weather started Tuesday morning. During the cold weather (30ish with some rain/snow), I was getting fuel economy in the upper 40s. I am only about 6 miles from my work. After I get to work, the car sits for 4 hours and then I go to the gym about 4 miles away. The engine just never warms up. This means that the battery is not charging properly, the autostop is not working properly, and the engine keeps idling when I am coasting and take my foot off of the gas. Last night, I had a round trip with the car already warmed up. I was able to get upper 50s and finally got my tank average over 50.

I have noticed the drop also.

In addtion to the cooler temps already noted, I would add that reformulated "winter fuel" is now hitting the fuel stations, this always drops FE in any vehicle. As I was commenting to someone the other day - a 15% drop on the typical 20MPG vehicle = 3MPG, a difference yes, but certainly not as noticeable as 15% decrease off of 53MPG, a whopping 8MPG decrease!

Also, in my case, my Tires dropped from 39psi COLD down to about 36psi COLD due to the lower ambient temps. As of yesterday, I am now at 42psi COLD and have noticed a slight improvement.

For me, other causes of the drop include STRONG headwinds, and increased rolling resistance because of rain over the last couple weeks.

The mileage of regular cars also goes down with the cold and short trips.

You are still getting better MPG in a hybrid than a regular car.

Need fewer troops to support.
Drive a hybrid.
Best tank 71.65 MPG.

Before this car I spent two years learning hypermiling on my 2004 HCH1.

My worst tank last Feb was 40.0 mpg. I live near Buffalo, and the car was fairly new. I now have:

a. Tires pumped up to about 50 psi
b. A broke in car
c. A block heater
d. More HCH II driving skills

It shoud be interesting to see how I fare. Tonight I only lost about .1 mpg on the way to work (7.2 miles). Last winter I was seeing losses of 1 to 2 mpg on my work commute. It was about 45 F. tonight, and block heater was on for about 6 hrs. I don't have a garage.

Don
Best tank 67.5 mpg (555.5 mi.)
Best trip 91.5 mpg (42.2 mi.)

I'm from central Illinois, and I too noticed the sudden drop in FE. I thought I was getting pretty good at achieving 50+ mpg. After filling up late last week, I was only getting 36 mpg! I've struggled to pull it back up to 44 mpg.

I was considering getting a block heater, but I'm not sure that it would pay for itself...

Brad Barker Honda in Bloomington, IL quoted my $114 installed. That didn't sound too bad.

I didn't realize they were that cheap. I just assumed it would be a lot more expensive than that. I will have to check into getting one.