From what I've researched (no references right now, sorry), it's mainly air drag and tire friction. From 100F to freezing, air drag increases 10% and tire rolling resistance doubles. Using the
calculator, for the average car that means ~30% lower mpg at 35 mph and ~20% lower at 70 mph. You can add on top of that any extra warmup, lube friction, lower tire pressure if you don't keep up with it, etc.