Warm air is generally more fuel-efficient than air conditioning. This is because warm air utilizes the heat already present in the vehicle, whereas air conditioning requires energy to cool the air, which can significantly increase fuel consumption.

The reason warm air is more fuel-efficient lies in the mechanics of how each system operates. When using the heat function, the vehicle's engine warms the air before it is circulated into the cabin. This process does not require additional energy from the engine, as the heat is already being produced. In contrast, air conditioning systems use a compressor to cool the air, which requires energy that is drawn from the engine. According to the U.S. Department of Energy, using air conditioning can increase fuel consumption by up to 25% on a hot day. This is due to the fact that the compressor adds additional load to the engine, which in turn requires more fuel to operate.

Data from the U.S. Department of Energy further supports this claim. A study conducted by the agency found that for every 10 degrees Fahrenheit the air conditioning is set below the outside temperature, fuel efficiency decreases by approximately 7%. This highlights the significant impact that air conditioning can have on fuel consumption. Additionally, the study also noted that on average, air conditioning can increase fuel consumption by 20% on a hot day, which is a substantial increase in fuel usage.

In summary, warm air is more fuel-efficient than air conditioning due to the fact that it utilizes the heat already produced by the engine, whereas air conditioning requires additional energy to cool the air. This is supported by data from the U.S. Department of Energy, which shows that air conditioning can significantly increase fuel consumption.