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Everything MMJ said, plus...
Up to ~30 MPG you see better than expected results, after that you see less than expected results. That is the point where MPG ~= kMPG (thousand miles per gallon). The k puts the numbers in the same ballpark, and makes them more easy to understand vs our societal understanding.
The problem is that miles are the dependent variable. You do not fill up your tank, then decided how far you are going to drive. Most people drive a certain distance, and should think about how many gallons it takes to get there. We (the U.S. of A.) will adopt the metric system well before we change this "standard".
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