Who generally stated it?

You can't use a fixed number for all vehicles. It is a certainty that it takes more horsepower to push the vehicle through the air at a faster speed. This will consume more fuel. The only question is how much more fuel is required over the amount that is necessary to simply run the engine at that speed.

With a very economical engine that uses less fuel to run without a wind load, the wind affects the fuel economy in a significant way. My SD will lose about 10% on fuel economy if I change the speed from 62 to 75. However, the E38 doesn't seem to care if I keep the speed at 65 or run it at 80. I certainly know that it takes more fuel at 80 but the percentage is much smaller than the SD and the errors of fueling can easily lead me to a conclusion that it gets identical fuel economy at both speeds.

Fundamentally, I know that this is false and it consumes more fuel at 80 mph than it does at 65 mph, however it might be as small as 5% and I would definitely need to run at least 5 tanks consecutively to accurately measure 5%.

Otherwise, I might make a factually incorrect conclusion that it makes no difference in fuel economy if a large V8 sedan is run at 65mph or 80 mph.