Well, searching for my posts on this topic would probably help ...
It's really quite simple. The 2 main things that affect fuel consumption are the air:fuel ratio and the engine speed.
The lower the air:fuel ratio, the more fuel you consume.
The higher the engine speed, the more fuel you consume.
When you go faster, you must increase the throttle input to overcome the increased drag, which lowers the air:fuel ratio, which increases fuel consumption. In addition to this, you also increase the engine speed, which increases fuel consumption.
It doesn't take a genius to see that increasing speed is going to increase fuel consumption at a much higher rate, thus reducing your MPG ...
For maximum MPG, you would want to use the least amount of throttle possible in the highest gear.