Originally Posted by oldtamiyaphile
I think at altitude it's not so much the temperature but the density of the air. You don't want to do anything that reduces the density of the already thinner air.
Except when you do.
Warmer air = thinner air = wider throttle plate, which is good in every situation except where the throttle is wide open and you need to downshift for a bit more power. At part throttle loads, colder and denser air will cause economy to drop. Warmer air also (generally) helps with combustion efficiency.
Fun bit, I seem to remember some of the old Honda CRX's having a factory flap which fed the engine warm air under light loads, and colder air as the throttle opened up. This gave the best of both worlds - economy when you need it, and power when you nee it.