So, if you have your AC on and have it set for a lower temperature, the AC pump will be work more, then you will burn more fuel than if you have the AC set for a higher temperature, does that sound right? :D
I think so. The more the compressor runs, the more power it's going to use.
At some point, if the weather is hot enough, the compressor will run 100% and still not be able to bring the temp down to say 70 degrees F. As I understand it, the A/C can only bring the vent temperature down X number of degrees below ambient.