No, the impact will be large even if you don't use the HVAC. Aerodynamic drag rises as temperature falls because the density of air rises. A tire's rolling resistance rises when the tire is cold. Headlight usage increases in darker months. Every single lubricant in the car becomes more viscous, and bearings and shafts will sap a little more power. If snow and ice are allowed to build up on the car, they will add aero drag and weight. An uncomfortable driver is a less effective hypermiler. I'm sure there are other factors, but those are the big ones. Defrosting the windshield is always interesting in the winter. I've always had trouble with that, until I got a place with a garage and started parking with the driver's window down.