Does the driver start the car and let warm up for more than a minute? Warming up a car long term = 0 MPG

I recent got a non MB with an average MPG readout. It had last been driven round October 2017 ( warmish weather ) . MPG reading was 23.8 , I started driving it early Jan and MPG started to drop instantly. After a few weeks of 0 to 7 * F mornings and 12 * days. MPG settled out at 18.5. Now that it is getting warmer, MPG has risen to 18.9.

A very cold engine takes longer to warm up so it spends more time in fuel enrichment. Oils are thicker causing more drag, especially rear end oil. Driving through snow creates a resistance too.

Running the rear defroster? Heater blower on high? These take more energy as well.