Why would improving aerodynamics/drag/weight/etc. affect mileage?

[imzjustplayin]

When I say these things, I'm saying this in a very specific manner. I'm under the presumption that the engine speed and fuel consumed is at a perfect ratio to that of the transmission. If you're in 5th gear and the engine is spinning at 2000rpm, it should consume X amount of fuel and be moving at a specified speed, always. I'm aware of load sensing and whatnot but I'm still confused about what I'm going to ask.

If you were to improve the aerodynamics, the drag, the weight, etc, making the car more "optimal" but you left everything else intact, when running at a constant speed, wouldn't your mileage be the same? Doesn't the engine consume the same amount of fuel regardless of the load?