Originally Posted by
PityOnU
*adjusts glasses*
Actually, warm weather increases MPG by reducing the density of the air, which reduces wind resistance/drag. There is also less time spent by your engine running rich trying to warm up. Also also the decrease in air density reduces the amount of oxygen your engine is able to suck in each cycle, causing your ECU to reduce the amount of fuel it injects to maintain the oxygen/fuel mixture ratio.
The latter point also makes your engine less powerful in the summer, and is also why cold air intakes (actual cold air intakes, not the dumbass under-hood pipes companies end up calling "cold air" intakes) and nitrous can increase engine power output. It's also why some members on the forum have intentionally put "warm air intakes" on their cars.