Excellent calculator that returns values close to my own:
The blue line is what the calculator appears to be generating. My gray line includes the fixed vehicle overhead, the 'heart beat' cost of having the vehicle on. Thus at 0 mph the vehicle achieves 0 MPG.
The only element missing from the calculator is vehicle overhead, what it costs if the vehicle is turned on and not moving (aka., idling overhead.) What this does is identify the maximum range speed and expected MPG. The calculator is fairly accurate at speed ranges where the vehicle overhead is much less than the rolling and aerodynamic drag.
Performance models are great because they give an idea of what the vehicle should achieve. To a greater or lessor extent, I can evaluate changes with the model and verify them on the road. But it is very important to go on the road and plot the mph vs MPG. The gaps between the model and field measurements can identify "sweet spots" as well as 'black holes' and leads to insights about our vehicle systems.
Bob Wilson