how cold should it be

 

Depends on outside temp and humidity, but if you are going down the road and getting an actual 38-45 degrees fahrenheit out of the center vent you will probably be comfortable. Those red pocket dial thermometers that you get at the auto parts store often vary 5-10 degrees, so it's hard to get an accurate reading. What they are good for is monitoring change. I keep one stuck in my vent all the time, solely for that purpose. To get the actual temperature out the vent I use a higher quality thermometer that I got at a liquor store, which is used to measure the temp of wine. It has better response time and a 0-120 degree scale, which I think is preferable to the 200 degree scale on the cheap ones. Bear in mind that if you get stuck in traffic on a hot day the air won't blow as cold as it does when you are going down the highway at 70 mph.

 

When the ac place is charging my car, their goal is 54 degrees.