Your turned the A/C on..??

Now the ICE must provide HOT coolant for the HVAC's reheat/remix cycle after the A/C cools the airflow down to near freezing.

You can improve the efficiency of the A/C (and thereby FE) quite dramatically by running it in MAX COOL mode and then using the blower speed manually to regulate the cabin comfort level. It wouldn't hurt to purchase and install a water flow shutoff valve at Home Depot to stop the flow of HOT coolant to the heater core.

The ICE will still run occassionally to keep the coolant (and catalyst) hot but not as often with the shutoff closed.