The 55mph speed limit and fuel economy has come mostly from politics. There are many factors that affect MPG such as elevation tempature and if the car is going up or down hill. There is no "one" speed that gets optimal MPG unless your driving on a flat road or on a dynamometer. The car's shape coefficient of drag and engine size would affect this as well.

I do a bit of flying in private Aircraft and their GPH figures are not based on formulas. When the plane is in its test phase pilots from the test firm will take the plane out and do several hundred flight tests and the data from these tests are logged and put into a graph which is a rough at best guide to the Aircrafts performance. In aircraft engine performance you get a much larger range of conditions that the aircraft is subjected to. When you go from sea level to 10,000 to 20,000 feet within a matter of minutes your engine performance and fuel economy changes fast. Pilots have to keep up with these variations in real time to make sure that they have enough fuel to complete their intended route. Some smaller private planes based at airports in higher altitudes can be grounded by variations of barametric pressure. When the pressure changes these planes don't have enough power to safely take off on a certain length of runway.

Personally I wish car makers would realease the inforamtion like aircraft makers do. However in our politically charged world with lawsuits being filed for any reason its too much of a liability. If your car has an optimal MPG at 75mph and you put that in the manual and your customer gets in a deadly accident your going to get sued. Im sure Mercedes has done tests that could be converted into a graph to show you your optimal speed vs mpg but that information is locked in a filing cabinent somewhere and will never be released due to the sue factor and the lack of demand for the data.