Fuel Economy in Hot Weather

Hot weather can actually increase your fuel economy. Your engine warms up to an efficient temperature faster; summer grades of gasoline can have slightly more energy; and warm air causes less aerodynamic drag than cold air.

However, keeping passengers comfortable in hot weather by rolling down the windows or using the air conditioning (AC) can reduce fuel economy.

Running your car’s air conditioning is the main contributor to reduced fuel economy in hot weather. Its effect depends on a number of factors, such as the outside temperature, humidity, and intensity of the sun. Under very hot conditions, AC use can reduce a conventional vehicle’s fuel economy by more than 25%.1,2 The AC’s effect on hybrids, plug-in hybrids, and electric vehicles (EVs) can be even larger on a percentage basis.2

Driving with your windows down can also reduce fuel economy. Open windows increase aerodynamic drag (wind resistance), making your vehicle use more energy to push through the air. This effect is quite small at low speeds but increases at highway speeds.

…more at http://www.fueleconomy.gov/feg/hotweather.shtml