Hot weather can increase fuel economy - the 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 comfortable in hot weather by rolling down the windows or using the air conditioning (A/C) can reduce fuel economy. Running the vehicle'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, A/C use can reduce a conventional vehicle's fuel economy by more than 25%, particularly on short trips. The A/C's effect on hybrids, plug-in hybrids, and electric vehicles (EVs) can be even larger on a percentage basis.
Driving with the windows down can also reduce fuel economy. Open windows increase aerodynamic drag (wind resistance), making the vehicle use more energy to push through the air. This effect is quite small at low speeds but increases at highway speeds.
However, opening windows to let out hot air from the cabin is helpful. An especially good way to do this without seriously impacting fuel economy is to vent the sunroof while first driving. It not only lets out the hot air but helps keep the vehicle's aerodynamics at the same time..
How To Improve Fuel Economy in Hot Weather
- Roll the windows down at lower speeds; use the A/C at highway speeds.
- Don't use the A/C more than needed or set the temperature lower than needed.
- Park in the shade or use a sunshade so that the cabin doesn't get as hot.
- Drive with the windows open for a short time before using the A/C. Letting hot air out of the cabin first will put less demand on the A/C and help your vehicle cool faster.
- Don't idle with the A/C running before driving. Turn the A/C on after starting to drive or after airing out the cabin briefly. Most A/C systems will cool the vehicle faster while driving.
- Read the vehicle's owner's manual. Most manuals explain how the A/C system controls work and how to best use and maintain the AC system.
- For plug-in hybrids and electric vehicles, pre-cooling the cabin while plugged into the charger can extend the vehicle's range. Also, using a warmer temperature setting for the A/C will use less battery power.
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