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yes rolling resistance of winter tires is higher.Well, this certainly wasn't what I was expecting.
1. Winter formula gas. That usually reduces mileage.
2. Winter wheels and tires compared to all-season Continental tires. That should reduce mileage in the winter.
also air density is higher in winter.
aerodynamic drag equation
F=1/2 CρAv2
where rho is air density. drag is proportional to density
also P = nRT/V
pressure is proportional to temperature.
air density changes by about 5% every 30F. therefore aero drag changes by 5%.
so that'd account for a mpg or 2
also, higher density cold air means that the throttle has to open less to get the same air, thus increasing pumping losses which decreases efficiency.