1989 Chevrolet Celebrity CL 4 door Gas Mileage (Red Car) - EcoModder.com
My fuel log shows the same thing, a considerable drop right at the beginning of October. I can't say it's due to weather and temperature, since most of the summer was pretty horrible for weather and most of the fall we have have a lot of the nicest days of the year. It could very well be winter gas.
I do agree that my best mpg came from the handful of 70-80F days we had this summer. I even got 40 mpg going home from work 2 or 3 times.