Volkswagen Golf GTI 2016, Owner'S Manual, Page: 831 / 1341

Climate control system (air conditioner): If the air conditioner has
to produce starkly contrasting temperatures, it requires a large
amount of energy, which is generated by the engine. The
temperature in the vehicle should therefore not be extremely
different from that of the outside temperature. It may be helpful to
ventilate the vehicle before driving and then to drive a short
distance with the windows open. After that, switch on the air
conditioner with the windows closed. Keep the windows closed
when driving at high speeds. Open windows increase fuel
consumption.
Switch off seat heating once it has served its purpose.
Switch off the rear window defroster as soon as the windows are
free of fog and ice.
Additional factors that increase fuel consumption (examples):
Malfunctioning engine control.
Driving in the mountains.
Towing a trailer.