I tried to fight my nature and just estimate the tail end of the coast down curve, but I finally gave in and made the trip over to my test area and reran things starting at 35 mph. It didn't flatten out as much as I expected, so maybe that was a good thing. I merged the high and low speed runs in this graph. I pseudo-coded the program I need so I should have the results soon.
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The Elantra is an AT car, 2006. The other two are MTs, with the Nissan being a 2002, and the 2006 Scion being an aerodynamic barn door.
The highest mpg values in the upper left are all P&G values. The dips from the P&G values in the respective lines are the results of lower gear runs. The two MT cars I can verify. I'm guessing that's the reason for the AT Elantra's mileage dip.
The AT auto simply isn't in the same league as the others for high mileage at lower speeds. I can't FAS with it, so it's always running at idle speed or faster. For much of the graph, its mileage trails the MT cars by 10 mpg, and it doesn't catch either of them until 60+ mph (speeds I don't typically drive at).
The interesting thing is how flat the mileage drop-off is on the Elantra compared to the other two. My guess is the Elantra is much better streamlined, as its gearing is about the same as the Nissan's. They both churn about 2200 rpm @ 55 mph, while the Scion buzzes at 2600 rpm at that speed. The Scion beats the Nissan's mileage at about 62 mph. If I extend the lines, the Elantra should best the Scion's mileage at 72 mph and faster.