The figure is from a master's thesis of an MIT student. The author simulated optimal fuel economy of four vehicle models using the Powertrain System Analysis Toolkit (PSAT) developed by Argonne National Laboratory (ANL).
Fuel consumption versus cruise speed for steady-speed driving is simulated as the following figure.
Maybe some of you are not familiar with the metric system (kph or l/km). When the figure is translated into mpg and mph, the optimal speed and fuel consumption of the four vehicles are (from a data table on page 65 of the thesis),
|Model||Optimal Speed Range (mph)||Fuel Economy at Optimal Speed (mpg)|
It's amazing. It's too good to be true. However, even if the simulation overestimated Civic's 71 mpg, it is evident that vehicles' fuel-efficient speeds are well below most highway speed limits (55-75 mph).
I had better avoid highways, when I am not in a hurry.
Source: Berry, I. M. (2010). The Effects of Driving Style and Vehicle Performance on the Real-World Fuel Consumption of U.S. Light-Duty Vehicles. Masters thesis, Massachusetts Institute of Technology, Cambridge, MA. Retrieved from http://j.mp/speed_mpg
1 mile = 1.609344 kilometers
1 gallon = 3.78541178 liters