"Climate control gets heat from the engine. The longer the engine stays cold, the longer your period of low mileage. Tom" Good point Tom - What about AC?
I took these measurements in the morning commute. I tried to repeat in the PM, but traffic was so bad, MPG was well below average. Miles/Cumulative MPG (rounded) 1/25 2/38 3/45 4/50 5/55 6+/60 No heater or AC, slow city driving (average 20 MPH).
That looks about right. An engine block heater will drastically shorten the time you are getting the "poor" mpg - from 3 miles down to just 1, for instance.