I noticed the same thing with mine. All of a sudden I got a 20% reduction in one tank. I am not sure what to think but I am waiting for a few more tanks before I get worried.

 

I seem to get a drop like that after every major service (15K, 30K, 45K and just last week 60K). I've speculated that they do some kind of service requiring that the engine idle for a time. That's just a guess, but idling will kill a tank. Have you gone multiple tanks, or is this the same tank you had when the car was serviced?

Dealers will also reset tire pressure to 32 psi if you ask them not to. Even then, many will anyway.

After driving for four years in my HCH (and four winters), there's a lot of variables that make isolating a cause hard to do. I take hits if it's cold, windy, rainy - take your pick. Sometimes it just goes down and I can't explain it. After four years, I'm currently on back-to-back worst tanks since I owned the car (40.5 mpg last tank, and this one is shaping up to be 39 ish). Amusing that I'm actually complaining about 39 mpg.

 

I have driven many many tanks of gas since that last service... holiday vacation, commuting, etc. Weather is pretty cold here in the mornings, but I keep the car in the garage. Some mornings I do let it warm up/idle for a few minutes, but never for long at all, and not most mornings.

I don't know about disengaging the A/C, I've read references to that... Obviously I'm not running it, but will it kick on as part of the defrost when the windshield is foggy and I use defrost to clear it? Is that something I can/should do? Still, this mpg is lower than anything I got in summer, when obviously I used A/C more regularly!

I need to check tire pressure... I didn't get a chance over the weekend, but maybe the cold weather (even though it's not bitter) has reduced that, and/or maybe the dealer did.

The last several tanks have averaged about 40 exactly (though fill-up/trip odo puts it a tad higher). I have not yet explored the database here, I'll do that...

 

On the A/C, if it's like the HCH I (and I think it is) you have no choice. Defrost automatically engages the A/C whether you want it or not. It's really frustrating in the rain (my car fogs up when it rains). So not only do I get to drive in the cold and rain, I get to run the A/C too.

On the tires, 10 degrees outside will raise or lower your psi by 1. So setting the pressure at 35 in 90 degree summer heat means it will be 30 psi in a 40 degree winter (does not count for normal loss either). Always good to check.

Here are two graphs of my mileage over 4 years. You can see in the white sections (which is the first quarter of any year - Jan - Mar), if you follow the yellow line (which is the average of the last 10 tanks), you can see the rise and fall across the seasons.

 

I don't believe the battery packs are as efficient at low temperatures. I've also noted a very noticeable drop in MPG when the temperature dips into the 20's or 30's -- even if I am not running the heater. It seems that the best mileage will always occur in the Spring and Fall - when the temps are not high enough to warrant use of the A/C and the battery pack is warmer. Note, in the summer, if you open the windows, you also lose MPG because you have introduced more drag by airflow into the car.