The short answer is yes, car air conditioning systems do affect cars’ fuel consumption. However, the actual increase in fuel consumption depends greatly on several factors. These include the following-
The size of the cars’ engine
As a rule, smaller engines are more affected than bigger engines. Bigger engines develop more power, so the parasitic power loss caused by the compressor is lower than on small engines.
How often you use the air con system
If you run the air con system for ten or twenty minutes each day, the effect on fuel consumption will be smaller than if you run it for two hours each day. Thus, if you only need cold air for a short while, switch the air con system off as soon as you can to save fuel.
The temperature you set
The lower you set the temperature, the longer the air conditioning system has to run to achieve that temperature. Therefore, you can save a lot of fuel by setting the temperature to a comfortable level, instead of "icing" levels
The design of the compressor
Variable displacement compressors were introduced specifically to reduce fuel consumption. Because they are more efficient than fixed displacement compressors, they reduce parasitic power losses. This improves fuel consumption, and creates less harmful exhaust emissions, at the same time.
Unfortunately, there is no easy way to calculate how the air conditioning system in your car affects your car’s fuel consumption. If this is important to you, we suggest that you fill the tank, and use the car and the air conditioner normally for about two weeks.
Then fill the tank again, and use the car normally for a similar period, but without using the air conditioner. If you fill the tank again, you should notice a decrease in the cost of filling the tank. This difference should give you a good idea of how the air conditioner is affecting your car's fuel consumption.
It should also give you ideas on how to better use the air conditioner to reduce its effects on the car’s fuel consumption.