Cars Q&A

I own a Hyundai I-10 for about 2 years period. I want to get myself clarified on following Driving habits : (1) What should be optimum RPM with regard to Gear and if the Car is driven at lower or higher than optimum RPM what are the disadvantages. (2)In the case of A.C.: Does it make any difference on fuel consumption if A.C. is run at the highest cooling or if the fan speed is kept at the maximum .