Can someone explain this to me

The point I attempted to make got lost in headwinds and tailwinds apparently. Try something like this for yourself:

When entering a freeway, floor your gas pedal (lets face it we need to) and do that acceleration up to the traffic speed, without thinking about fuel burn (it is burning indeed). Mpg goes down down down for this interval. If you want to go faster then keep the pressure up until you have reached the desired speed. It is not being said to do this gradually. You just abandon all thought of fuel economy until you reach the desired speed. Then you can lift your foot off the gas pedal and begin to coast.

Compare it to the ascent of an aircraft. It is all throttle until the altitude. In the driving experience the equivalent is the desired speed. Maintain the speed throughout the length of the freeway taken. But the cavaet is about how to maintain the speed and build your mpg. Try this for how to do that:

I use the Eco Score screen for it. I have direct visual feedback on it. The arch that has Eco on the left and power on the right, is feedback that proves itself with every use. I use this as my guide for knowing how much pressure to apply to the gas when I periodically use it to keep the speed up. The graphic should be as far left as possible for the maintenance of speed. It is coasting and then nudging.

That is how it has worked for me. You burn as much fuel at 80 as you burn at 65.

 

If you're going a constant speed on a level road without any wind and there is no difference (such as standing water) in the rolling resistance, then NO, you DON'T. Unless by "as much fuel at 80" you mean "at least as much fuel at 80".

Your description of applying a certain amount of throttle is inaccurate for several reasons. First, the engine can be spinning at different speeds even with the same throttle input and that will affect fuel consumption. Secondly, the drivetrain can be one of several configurations based on driving conditions, and that will also affect fuel consumption.

But even if we assume that a given throttle position equates to a certain amount of fuel flow, the speed that you're going can vary, and that will also affect fuel economy. The throttle position needed to maintain speed at 65 won't be able to overcome the increased aerodynamic drag encountered at 80, and if you use that amount of throttle at 80 then the speed is going to decay until you're eventually going at 60.

 

Not true at all.
Wind resistance is the largest factor in fuel usage.
Not only do higher speeds increase fuel usage, it puts more strain on engine components.
And the faster you go, the more it hurts if you come to an unexpected sudden stop (crash).

 

To everyone. Sorry your experiences have been more disappointing than mine. Yes. I had the MAGIC Prius C.