Diesel gels in winter. Gasoline doesn't. But gasoline may freeze with a high water content. I think the winter blends of gasoline have a slightly higher ethanol content.



Cold air intake charge promotes better combustion. But there probably is a point where cold is too cold.

The main reason for less mileage in winter is due to the engine running in open loop longer until it warms up. In this mode the O2 sensor data is ignored by the ECM and the ECM forces a richer fuel mix that a cold engine requires to run decent. The higher RPM cold idle is a dead give away that the engine is in open loop. The concept is identical to the old technology carburetor and choke. Once the engine warms up the ECM switches to closed loop mode where the O2 sensor trim is used. Also, drag is a greater factor in cold weather. The 90W gear oil is like thick molasses until it warms up. Same for the wheel bearings. They all contribute to a lessor MPG.