We are having an ongoing heat spell here in Orange county, CA... and our MPG is still holding in the steady range of 36 mpg with our using the AC on low most trips. We aren't getting near 115 degree heat (I'd MOVE from anywhere that got that hot!) but we are getting about as warm as you'd ever expect for my area.

It NEVER gets very cold in Southern California. What is the cold weather range that can start to affect MPG?