Temperature plays a huge part (and I have decided not to worry too much about reaching HyperMiler status - too cold in Chicago to get good mileage right now). But distance also plays a big part. My last tank dropped to 40, after many tanks all summer long over 50 (And it dropped me under 49 LifeTime)

I drive 40+ miles a day each way to work. I can still get 50 MPG once the car is warmed up, but it takes so much longer when it is 4 degrees outside (and worse if the car sits outside instead of in the garage).

My friend drives a Prius, and her mileage suffers as much or more in the winter than my HCHII, I think because she drives 10 miles to work and her car never warms up on the commute.