That could be acceptable depending on ambient conditions such as outside
temperature, humidity, amount of sun load, weather you are idling or
cruising down the highway at 55 mph, and length of time that the unit was
run before taking a measurement.  If you are getting 48F in the sun on a hot
humid day after 15 minutes with the fan on high speed while idling, then you
are doing very good. If you can only get down to 48F at night on the highway
for an hour and the humidity is not too high then your system could probably
use some service. Oh, and it should get colder if you have it on Max A/C
rather than fresh air, and it will be colder at lower fan speeds. So you
see, it all depends.