Originally Posted by Commander Keen
The problem I'm seeing is that the engine sometimes struggles to maintain operating temperature, which definitely has an impact on efficiency.
Most of the engine's power goes towards moving the air you drive through. In the winter, this air becomes more dense and provides more resistance.
If the thermostat is working correctly the engine should stay at operating temperature without any problem.
Moving the air (wind resistance) is in fact generally the largest factor in fuel consumption rate, once you're warmed to equilibrium. If it's 50% (and that will be determined by the aerodynamics of the vehicle) and density is increased by 10%, you can expect your fuel consumption to increase by 5%, all other things being equal.
As far as colder air intake, the increased available power from the colder, more dense mixture going into the combustion chambers is offset, at steady state, by needing less weight on the accelerator pedal. It's the change in density, not temperature, that affects the available power.