The difference between 70 and 80 mph is quite surprising, something to do with the maths of air resistance increasing non-linearly with speed. In short, you will squeeze a lot more out at 70 than 80, even more at 60. I can only presume that this is why I often notice EVs on the inside lane being driven by owners sporting imaginary halos or the look of fear on their face as they dice with it to make it to their next charger.
Just done a rough and ready approximate plot of speed versus power needed to overcome aerodynamic drag for the Model 3. I used a projected frontal area of 2.22m² and a value for Cd of 0.23. The plot shows clearly how the power needed, and hence drain from the battery pack, rises with speed: