You've got distance and time to factor into the equation.
Are you saying that you are trying to balance fuel consumption versus your own time? I find the easiest graphs to read the ones that compare fuel consumption/power versus rpm. On most diesel engines, this will occur near the torque peak. This can be manipulated a little through things like timing but not much. At high rpms (which I would consider 80% to be), you are putting a lot of energy into simply spinning the engine which is also not running as efficiently. 80% is perfectly acceptable if you want to get somewhere relatively quickly but it is not going to yield optimal fuel economy.