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ok, so everyone was complaining this summer about how hot it was and how it affected the cars. so ive been wondering, now that its gettin cold(here in ga anyway) how much of a difference in mileage and performance should we expect? better throttle response? how much does temp really affect performance of a motor?

 
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engines will be running at the thermostat set temperature range. regardless of outside temp, the engine itself will be constant.

only way i can see this doing squat to anyone's engine is if they have a turbo, where the intercooler air temp is highly affected at ridiculously fast speeds (air temp drops 1 degree for every 8mph of air speed). and even at those speeds, the differences between the same civic in two different temperature settings (assuming clear day, good traction, no inclement weather, just cold as hell, compared to the same track on a hot day) would be barely noticeable if at all.

air temp probably affect your tires most on a civic

 
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Good question.
I don't have yet MPG for cold period. I du know the altitude have not influenced my MPG (one trip from 0 to 7000 ft). The temperature was around 40 F. I got same MPG (44) as usual.
Maybe some member from Canada can share information about civics performance in cold climate.
The big isue is how much time takes to heat up the engine, because after that the ECU is in closed loop.

 
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usually every 10F drop in temp gives 1% more hp. So if you go drive in 90F weather and then come winter it goes to 10F, you potentially gained 8whp. So yea it will feel a bit faster due to the air being more dense (more oxygen per given area of space). That means your engine is getting more air, and the ecu adjusts by adding more fuel.

 
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More hp
So wors MPG

 
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I think something else you have to take into consideration is altitude... Here in Denver we're 5,280 ft. (mile) and when you take a little drive into the mountains you can easily be at almost 10,000 ft. In the summertime, cars can overheat due to the heat and thin air.

 
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I don't know about overheating isues at high altitude. At 4000 ft and 124 F on dirty road in Death Valley, using first and second gear for 5 hours no overheating. A/C on all the time.

 
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r u sure these ecu adjusts on all fuel level and rpm steps? i thought 7th gen computers only really adjust fuel levels at constant speeds (leaning it out a little) to grab some extra gas mileage.

air that is about to enter a CAI is hotter than the air surrounding the car, and the air that is about to enter the intake manifold at the end of the journey thru hot intake piping is even higher temp... so the surrounding air doesn't add a strict 1hp per 10F drop, its probably more minimal. that would be more applicable to a top fuel dragger where the air is stuffed down with RAM intakes and supercharger where air temp really plays into engine performance

 
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the ecu adjusts +/- 20% in close loop mode (all pedal positions except wot). yes it won't be exactly 1hp per 10F, but everything stays alot cooler. I can go for a drive and even after the engine warms up, alot of stuff in there is still cold or just mildly warm.