Well the 5% came from searches on the internet. Most sites say that 55 is the best speed for MPG. Some sites have graphs that show the MPG dropping for higher speeds. Like everything you read on the internet there will be 100 or more difference answers. But the fact is, my 500 will only get 20 mpg or more if averaging 70 plus. This is a proven fact and I have the spread sheets to prove it.
I think that the CD can be altered by the designers to tune the car for higher speeds. I know the engines can be designed for a specific HP and torque at a specific rpm range. So I don't think there is an answer to my question using readily available information. I am going to try some coast down hp test and see what happens. Maybe that will tell a story.

Paul

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84 500 SEL (307,xxx miles)