That info is a little 'watered down' - The efficiency curve looks awfully suspicious as it shows peak at 55 - 60 - the most common speed limit.
Peak engine torque is not necessarily peak efficiency - iirc the EJ20 has to run pretty darn rich to put out that high torque number so while it's making more torque/power it is not necessarily at peak efficiency. How much fuel do you ahve to dump to get that torque? Also, the peak efficiency of the engine alone is not enough - it's the whole car we are concerned with.
What you are trying to optimize is:
Maximize the distance travelled
Minimize the drag - both parasitic and drag related
Keep the engine speed in it's efficient range