fuel delivery is almost fully dependent on the degree of throttle position, especially in fuel injected cars. In Neutral your car is electronically programed to provide a specific amount of fuel to keep the car in an idle state, when in gear the ECU of the vehicle is programed to provide a specific amount of fuel depending on how open or closed the throttle is. Thats why you notice that if you gas a car in neutral that your RPMs will dip a bit before returning its programed IDLE RPM.

You will actually get better gas millage in a lower gear at lower speeds than you will at a higher gear, it varies car per car, but for optimum MPG the RPM position should almost always bet just slightly bellow the RPM of where the torque peaks. Because at that position, the force required to keep the car moving in relation to the resistance applied by the transmission will require the throttle to be open much less then it would at a higher gear but lower RPM. This is true for at or around the peak of the torque curve, because after that more force is required to maintain the higher RPMs

For example, traveling 35mph in 4th gear vs 5th. Say your Honda Civic's torque peaks at 3800-4200RPM, in 4th at 35MPH you will be pushing around 3200-3500 RPM, while in 5th gear you will be pushing more like 2600-2800RPM. To keep your car consistently at 35mph requires X amount of force to the wheel. ( I dont know the specific formula for calculating for perpetual velocity) X force to the wheel is achieved by calculating the force produced by the engine, and applied to the resistance of the given gear (weight plays a roll as well, but then you have to factor in inertia and friction). In order for an engine to produce more force it must burn more fuel, so the force necessary to keep the car moving at 35mph at 3500RMP will be less than the force required at 2800RMP (in respect to the torque curve), because it will require the throttle be to open at lesser degree.

Just look at it this way, if you are in 5th gear going 20MPH, and you mash the gas all the way down, even though you car is not accelerating as fast as it would going from 1st-5th, you are still dumping relatively the same amount of gas in to the engine. (especially true for cars with Carbs, as been said modern injected cars have dynamic ECUs, which will actually limit the amount of gas being injected as to not flood the engine)