To what degree does running the A/C in a car decrease the fuel efficiency?

Question

and to what degree does running the A/C at a lower temperature and/or fan speed decrease the amount of fuel consumed?

Answer 1

It does not decrease it at all in anyway no matter what temp you use it at. What does decrease your mileage is driving with the windows down because the air pushes in threw the windows and causes restricted air flow causing the car to work harder. Hope I helped you with this info I was told this by my mechanic.

Answer 2

it depends. if you have a large compressor that puts a lot of load on the engine, it will use more fuel. it will also cool the cabin quicker. if you have a very small compressor it will use less fuel but it won't cool the passenger compartment as effectively.