The snowflake's got me: How low can it (MPG) go?

Forgive me if this is a newbie question. My Prius and I are going through the first winter together.

Up until the end of November, I'd been averaging about 50 MPG lifetime (since July). Now we're into December, and it has started to get cold. My current tank is showing about 45 MPG over 300 miles (so far). Temps have been in the 20s - 30s for a couple of weeks. Since it will eventually get significantly colder still (usually we have a few weeks in the 0s - 10s in Jan-Feb, I am wondering what kind of mileage I can expect.

Has anyone made any statistical observations on the effects of cold weather on Prius mileage averages? For example, can a 10% MPG decrease be expected? 20%? more than 20%? If you're one of the lucky folks going through a second winter in your 2004 Prius, and would like to post your observations, I'd appreciate it.

Thanks,
G.

 

Thanks Evan and John.

20% seems a bit harsh, but it is in line with the trend I'm noticing.

I recall noting that my previous vehicle dropped from about 17MPG to 15MPG, right about when the winterized fuel hit. After that it was pretty much temperature agnostic. That was only about a 12% decline in MPG, but it wasn't nearly as optimized for economy as the Prius. :mrgreen:

As for tire pressure, I'm pretty religious about that sort of thing. It was the first thing I checked when the weather started getting colder. I've seen info stating that the average tire pressure drops 1 PSI for every 10 degree F temperature drop (and conversely gains 1 PSI for a 10 degree F rise in temp). As a sidebar, one reason I am so tire inflation aware is that I traded a Ford Exploder on the Prius, and managed to survive driving for three years on Firestone tires-o-death.

Glenn