Originally Posted by Diabolus Ex Deus
I Simple mechanics. Gearing, etc comes into play for sure but I still say my comment is relevant.
Perhaps, but all the variables come into play only if comparing different vehicles. Two identical vehicles should have roughly the same MPG characteristics, but that isn't necessarily the point. Well, perhaps it is, when the claim that all vehicles get poorer fuel economy numbers above 55 MPH.
Perhaps looking at the original claim, and asking a somewhat different question. Are there any automobiles which get better fuel economy numbers at steady state speeds above 55 MPH? If 55 MPH is the fastest speed for the MPG peak, then there could be a claim that all vehicles get poorer fuel economy numbers at speeds above 55 MPH.
I think it would be nice to know if there really are vehicles which get better fuel consumption numbers at, say 70 MPH compared to 55 MPH. I realize that we will probably never return to the 55 MPH, and the typical 65-70 MPH limits are a concession to the somewhat higher fuel consumption numbers with the slightly shorter trip times.
I would suspect a detailed true speed to MPG graph would not be a smooth bell curve, but perhaps a series of mini-bell curves, as the engine speed goes from the lowest speed in each gear, an increase in the MPG vs speed for the gear to the maximum MPG and then back down as the engine RPM exceeds the RPM for that gear and can operate in the higher gear. Finally, in the top gear, there would also be a peak in MPG and then on the downside, as drag and internal friction losses increase to the maximum speed of the test.