FWIW, I don't find a seasonal variation, it's strictly a temperature-based. Data from last summer and this past winter, with Micelin Energy Savers in summer, X-Ice 3 in winter.
I'm seeing about 3 MPG per 10F temperature effect at an average mileage of about 45MPG, so it's about 7% per 10F. Surprisingly linear, too!
Have fun,
Frank