It is highly unlikely that a vehicle's average mpg would be cut in half, just because the vehicle got older. It is more likely that the mpg differences were the result of different vehicle usage. Perhaps the high mpg figures were observed when the Excursion was used on long highway trips with a light load, while the low figures may be the result of stop and go driving with heavy loads. Deferred maintenance could also result in a less efficient vehicle.
I've noticed smaller decreases in mpg in a couple of vehicles that I've had for a number of years. I attributed that to the increased percentage of ethanol mixed into our gasoline, since a gallon of ethanol will give you less mpg than a gallon of gasoline.
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