Winter Mileage?

Hello Everyone -

I got my 04 Prius in June. I have about 12K miles on it and enjoy it immensely. I have a question to pose to anyone out there who has had a similar occurrence, because the dealer doesn't really understand.

My summer mileage on this car averaged 46-47, mostly highway, and of course running the heck out of the A/C here in Virgina/Wash DC. Come fall, I had a best tank mileage of 51 - again highway mostly, with some off the beaten slow parts, and not much A/C. Okay - that all seemed normal to me.

Now - come October - usually wonderful weather in the 70s around here. But, my mileage started to drop off considerably. I am now at January 05 and running about 40 mpg per tank.

My theory - there is a direct relationship to the engine running, trying to warm up my butt on these colder days, and the low mpg -right? But, the October / Nov weather here was great, not much call for heat, so what's up? All I can GUESS is that the additives added to the fuel around these parts, for winter driving emissions controls (oxidizer?), may be the culprit.
I also noted to the dealer that the IC engine seems to start rougher and really do a nice stumble on some of the shut-offs (like when I pull to a stop light, just like a dieseling affect - yet using different grades of gasoline has no effect).

Anyone had any experience or knowledge on this? There seems to be no dealer found problems, and they say they get similar complaints from others, but find no issues with the Prius systems.

Frustrating!
Thanks.
Gene

 

Wow - I think I want to move to a warmer climate! I guess when you get a car that averages 16-20 mpg, you don't see a 10-15% falloff in the winter. I'll have to do a better job of checking my other 4 cars. I am still wondering if our colder climate fuel mixtures are the culprit or not!