The speed of your boat at deferent RPMs depends an what kind of prop you have and wind direction. Both will change your speed and fuel consumption.
To get the speed right use your GPS for land speed or I sometimes use my paddle wheel to get my water speed if in a strong current. As for prop. pitch whatever works for ya. Some go with a higher pitch prop and a beaver tail or drift sock to slow the boat when trolling. Some go with low pitch prop to loose speed but remember you loose on the top end too. Its really what you want. If you only drift or cast you probably want the higher pitch prop.

Hope this helps ..... Tight lines to ya!