We just got back form a 2k+ trip from Tampa to Dallas and back. We ran the roof AC whenever we felt the dash air was not keeping up with the heat. Usally 10 AM to 3 or 4 PM. I ran 60-65+ on the way out with no trouble and the AC worked fine. I would not worry about it unless your AC is acting up while you drive. I think there is enough air flow in general at that speed that it would work without the fan.
