Click on the picture above to expand the spreadsheet.
I'll spare you the economics lesson and simply refer you to the probably inaccurate, but wildly convenient wiki entry for diminishing returns.
And since I'm surely not the first person to point out the phenomenon, I'll also spare you an editorial on the implications of this in relation to the search for alternative fuel sources.
But I will say this: I did not create this excel spreadsheet (which you can download and experiment with if you like) for the purpose of disparaging or discouraging the research and development of hybrid/alternative fuel technologies.
I put it together for the simple purpose of satisfying my own curiosity into the practical savings one can expect with increased fuel economy, with all other variables remaining constant. And so it is only as an ancillary afterthought that I point out, as a matter of fact, that this spreadsheet happens to also highlight the diminishing marginal return from increased fuel economy.
Please note that this spreadsheet does not take into account any additional upfront cost for the increase to fuel efficiency, nor the ongoing costs associated with alternative energy sources necessary to achieve the absurd levels indicated in the spreadsheet.
Stated simply, what you see is a calculation, strictly, of the marginal savings (of money spent on gasoline) per year that an additional mile per gallon of efficiency would provide over a baseline amount.
Download the excel file and use your own variables