warm weather and MPG - HOT weather and MPG

[Phantom]

The only way I see it hurting MPG is that the air entering the engine gets to hot and cuts timing or the heat causes the engine to knock also cutting timing back.

[PaleMelanesian]

A hot engine / air intake reduces available power. There is one piece of my commute where it hurts. I have a right-hand hairpin into a steep climb. I can only take the corner at about 25 mph. In winter I can breeze up the climb in 4th gear. In the heat of summer I sometimes have to drop to 3rd. That burns an amazing amount of extra fuel.