An easy test to see if you should be up or down one gear.
Hold the throttle steady, slide your hand up so that you are also gripping the housing in front of the throttle so that you do not unconsciously move it. Now shift up or down 1 gear. Whichever gear ends up with the higher speed for that throttle setting is the gear you should be in for that combination of speed, loaded weight, wind, grade, road surface etc. If you are in to high of a gear you will actually have the throttle more open than if you were in a lower gear with higher RPM but a more closed throttle, thus better MPG. It is counter intuitive, but thee higher rpm in a lower gear the engine may be working less hard to maintain speed.