Averaging 28 as usual through the winter compared to 32 in the warmer months which is to be expected really
Longer on cold start / rich running due to cold temps
Tyre pressures drop in the cold increasing rolling resistance
Extra frictional losses due to all the fluids, oils and greases in the car becoming less viscous
Increased electrical load (heated seats, rear screen, lights etc) increasing the mpg
More aerodynamic drag on the car due to the increased density of the colder air
Winter sucks but cold dense air is officially turbocharger approved...