Sounds like you have a manual AC system on your car. You can emulate how automatic AC systems work:
- When cooling the car initially, use high fan speed with AC on, and have the airflow mix control set to coldest possible
- When the car has been cooled adequately, turn fan speed to low
- If it's too cold now, mix a bit of warm air into the cold air by adjusting the airflow mix control
Note that the last stage (mixing a bit of hot air) probably doesn't improve your fuel efficiency much (it can have a small effect on modern variable cooling rate compressors, but the effect is very small). However, turning fan speed to low may improve your fuel efficiency somewhat because most AC compressors nowadays have a variable cooling rate, and given such a compressor, it's wasteful to blow cold air continuously at a very great rate.
Me? I just start driving the car with temperature set to 21 degrees Celsius, because I have an automatic AC. The car computer does all of these adjustments for me.