If you're going to use your air conditioner most effectively, you might consider insulating your car and getting a temperature blocking film for your windows.
I have no personal gain in what choice you make, but in a quick Google search, I found: Vintage Air company sells aftermarket air units designed for the old car crowd and they list insulating products that can go above your headliner, inside the door panels, under carpet, etc. 3M's web site lists a window film material called Crystaline Auto Film that is made to block heat, not darken windows. This may make your car more comfortable without needing the air as early in the cooling season or as late in the cooling season. It could also make an A/C unit with a thermostat controlled compressor {if any car units do that} run less often. Good luck either way.
Personally, I run my Kia Rio's A/C only on the way home from work during the summer and anytime SWMBO rides along.
