[quote=efusco]Warm air for the intake helps mpg.
Warmer battery helps MPG.
Warm ICE improves speed of warm up.
Warm temps help keep the coolant in the thermos warmer.
Warm weather means summer fuel = higher BTU/gallon = less fuel consumption for same mileage.
Warm weather means less dense air to push the car through.

Warm is good!!

Yes..BUT:
warm intake air: bad...less dense...fewer moles of O2 to oxidize the fuel. Large diesels are intercooled after the turbocharger to help correct this. gas turbines are also more efficient with cool air for the same reason.

conversely, less dense air IS easier to puch through, as you note. So its a balancing act.

Interesting point on the summer fuel blend...

I suspect the "high humidity = high MPG" comes from the heat capacity of the water keeping the intake air cooler- the H20 absorbs some of the heat that would normally be heating the air and would make it less dense.