Variable displacement?

[nikos]

You will find that any savings to be had, come from the reduced available power.

If the powers there, and you flog it, you can get from 0-100 in say 5 seconds, reduce the total displacement by 1/2 and now it takes 10 seconds to go from 0-100kmh, you still get to 100, but you get there slower. The reduced rate of acceleration is what requires less fuel.

Now if we think about this, we could just reduce the rate of acceleration when running on all 8 cylinders too for an increase in economy, but because the powers there available to us, it gets used, and as does the fuel. - test it out for yourself.
Runing at 1/2 throttle is very similar to running at full throttle with 1/2 the displacement

This is where the savings are to be had, as if the powers not there, the fuel cant be used!

That is 100% true, but an internal combustion engine is not that black & white. A thing called "pumping loss" is what you're overlooking. Its all in this article.

http://en.wikipedia.org/wiki/Active_Fuel_Management