[SlaterSpeed]

Genrally in a cold climate your right. Its maybe not so clear cut in hotter climates and climates that are warm all year round tho.

On older cars you used to rotate the air intake of the engine over the exhaust in winter so the engine picked up hotter air and then in the summer you would rotate it away from the exhaust to pick up cooler air to compensate for a higher ambiant temp.

Im not relly an expert but its to do with the atomistaion of the fuel. For good economy you need the fuel to mix with the air a much as possible this makes the fuel burn better and more effeciently in the combustion chamber. The heat helps to keep the fuel and air mixed well (i guess this is simalr to the air evaporating the fuel)

However for power you need both good mix and alot of it. Theres a trade off between having a relly good fuel atomistaion and gettign dencer cooler air into the engine. and the trade off normaly favors the cooler air.