Here's some clarification:
Each individual vehicle is given a lifetime fuel economy figure calculated by total miles / total gallons. The model figure is then calculated using the median of the vehicles in that category. That way, a single car (no matter how drastic the outlier) will not skew the figure, a car with 100,000 miles will not matter more than a car with 50,000 and the final figure is as accurate as possible.

I put a lot of thought into it, and the median is the best way to go.

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