Jeff,

It's normal for the MPG to drop with the temperature for several reasons:

1 - The ICE (Internal Combustion Engine) has to run more often to keep it, the catalytic and you warm.

2 - Several friction items (eg oil flow) are higher at low temperatures

3 - the Traction battery loses efficiency at low temperatures.

There are others.

I maintain a spreadsheet based on miles driven and gallons used to track my MPG. (45.5 in my 2004 Prius over 70,000 miles). My MPG approaches 50 in the summer (in spite of AC) and is in the low 40's in the winter. I also find the reports from others suspect when they use less rigorous methods.

JeffD

ps: short trips really hurt MPG due to the warm up cycle.