Check out the hardcore posters in the database.
That would be anyone who entered data over most of the year.

Click on the name of their car.
First you will see a graph of their MPG vs. Month.

You will notice mileage is best during the warm summer months.
Then people from certain states will get a MPG hit during the months requiring air conditioning.

Why does cold weather lower mileage?

Need fewer troops to support.
Drive a hybrid.
Best tank 71.65 MPG.

Before this car I spent two years learning hypermiling on my 2004 HCH1.