Interesting plots, guys.
Andrew: re-label your chart with "month" instead of "temp". Then it's unassailable.
One only has to isolate a single variable pulled out experimentally to see the significant effect of temp on efficiency. This is a coastdown test of rolling resistance at various temps:
Source:
MARS projects - Crr vs Temperature
The data is for a bicycle/trike, but applies generally to auto tires as well. Anyone who pays close attention to efficiency knows they have to adjust their glide points on familiar routes as ambient temperatures swing, even with tire pressure and road conditions held constant.