Originally Posted by sanford12
The biggest reason for increased fuel consumption is more fuel used at start up in cold weather and more fuel used to heat and maintain operating temperature of the engine plus your drawing heat from the engine to warm the cabin of the car. All that is paid for with increased fuel consumption. All the other reasons mentioned add to the increase but over coming the cold engine is the main one.
Its ironic that as fuel efficiency goes up, it hits a point where the excess heat from the engine isn't enough to heat the car 'for free'.
With that said, i'm not sure that we're close to that yet. The excess waste heat from the focus's engine is probably enough to heat the cabin under almost any circumstance.
I did rent a tahoe once and going down from Taos Ski area back to the town, the engine didn't heat up at all until we hit the flats because the engine wasn't using any fuel.