Well, 40 degree air is pretty darned cold...I think the specs on the AC say if the air coming out is in the low to mid 50's you AC is working properly.
Now, a lot depends on the ambient temp outside. If you test this in the evening when it's cooler outside and get 45*, then the next day you get 55* in the heat of the day, it's because you are starting with a higher outside temp. The AC can only cool the outside air so much.
Check your Recirc. Door too. For best cooling, the door needs to be closed so it is re-cooling the already cool air...thus making it even cooler.