This is a graph showing what GM1kM, or Gallons pr 1000 Miles looks like compared to normal MPG.
MPG gives the illusion of linear fuel savings when in fact it gets exponentially more difficult to achieve.
The good news is that modest fuel savings on larger cars has a big pay-off in terms of efficiency. The bad news is that it will get harder to squeeze out that fuel savings the more efficient you make your car.
I made a
Excel spreadsheet showing the savings in gallons and dollars for every 2 MPG increase from your current MPG rating. You can edit your current rating, and the price of gas.