On Saturday, August 13, 2016 at 7:59:15 PM UTC-4, wrote:
:

Coldness "increases" at higher fan speeds because the higher fan speed moves a higher volume of air per minute from plenum into the cabin. The air itself doesn't get any colder - that is dictated by the compressor and the user temperature setting.

I'm speculating rashly here because I don't know how the automotive AC works.

But it's the internet so I'll think out loud based on a building AC setup.

There is no such thing as cold. There is only greater and lesser heat.

Dumping cold air into a space does not cool it UNLESS you are sucking warm air out.

In a building, there is a supply and a return. The return sucks out hot air and then the supply lets some cooler air move in and mix. No return, no cooling.

So MAYBE in a car, a higher speed will suck more hot air out of the space and cool it better even if the faster air over the coil is not cooled quite as much. At some point air must go over the coil too fast to cool all the way down to coil temperature (or heat up to coil temperature in the winter).. But that might not be the limiting factor.