Minnesota. What are the biggest factors in seasonal variation of fuel consumption? I think exclude driving style. What is left? My son and I were at a freshman (college) orientation last summer and a physics professor said as an aside, "who would think I could use my Pontiac Grand Am as a calendar. The graph of fuel consumption (mpg) was a perfect sine wave with a period of exactly one year." On my query, he stood on atmospheric resistance, 3% greater in cold and 3% less in warm weather. His observed fuel consumption varied roughly +/- 3%, lowest mpg in January, highest mpg in July. I asked about significance of fuel vapor pressure which varies by design on roughly the same calendar schedule. What say bitogers?