The 80 kph rule of thumb is actually part of the design parameters of most regular cars. They are built to be most fuel efficient at 80 (or probably more accurately aerodynamic designed for 80).

I was using exponential colloquially (and fair cop given its usage during Covid), but I think you’re just using cubic as a rough guide also due to air resistance. I’d note there are no extra gears at the higher speeds, so you’re probably less efficient on the tyres etc.