As the others have said, if you're looking for better MPG, warmer air is better in general. However, it will lower your horsepower a bit.
The general rule is a 5 degree drop in air temp equals 1% horsepower increase due the increase in air density. This means it also works in the other direction as well. Every 5 degrees means 1% less horsepower. Less horsepower increases engine load, and higher engine load for the same power output means increased engine efficiency.
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View my fuel log 2015 Mirage DE 1.2 manual: 47.2 mpg (US) ... 20.1 km/L ... 5.0 L/100 km ... 56.7 mpg (Imp)