It's interesting to see just how straight line your plot is MetroMPG. No Excel here, so no graphs but flattish roads have allowed a better idea of my envelope which would be typical of any overweight slushbox with piss poor aerodynamics.
MPG is almost flat between 38 and 50 MPH at around 32 MPG, and it drops sharply below that due to TC slippage if you try and drive steady state.
52 - 31
55 - 30
60 - 28
65 - 25
70 - 21
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