The ACFO study is interesting in that it seems to back up the notion that huge engines pretty much ignore the environmental variances that vex those of us with small engines. For example, the F250 with 460/5mt that delivered 13 mpg whether it was loaded, unloaded, into the wind or with it, slow or fast, hot or cold. NOTHING seemed to matter; 13 was simply what we'd always get.
P.S. A good part of that is simply percentages as in, +-5% is virtually unnoticeable at 13 mpg (12.35-13.65) but quite noticeable at 40 (38-42).
Come to think of it, there is much in the ACFO test protocol that confuses me. Were there many test subjects with very few data points per subject or few test subjects with many data points? Were things like filling errors and environmental variables like changing seasons considered? Due to the behavior of percentages as I noted above, the smaller-engined, higher mpg vehicles simply are going to experience greater fe variances no matter how tightly the testing is controlled.
Quote:
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Using data from real world tests on more than 500 model variants, Emission Analytics discovered that engines under one litre had the greatest variance from their official MPG figures -36%.
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It isn't clear to me if ACFO is saying the worst drivers got 36% less than rated or if the spread including everyone was +-18%.