ever figure out how much you actually save by increasing your miles per gallon? If it was from some additive, how far would you have to drive and how much gas would you have to buy to make up that difference? Well, here you go. It's more significant when you have lower fuel economy. The different curves represent what the increase in mpg is. So, regardless of what your current mpg is, if you increased it by 0.25 mpg, then use blue curve. If you increase it 1.0 mpg use the white curve. The X-axis is what your fuel economy is currently, so if my car gets 25 mpg and I wanted 0.25 mpg increase I would see a savings of 1%. So for 100 gallons of fuel used, at 25mpg = 2500 miles driven, you would save 1 gallon and only have used 99. Which is to say for 100 gallons of gas at $2/gal, you saved $2 out of $200 of gas. 1000 gal of gas over 25000 miles you save $20. The more efficient you car is to begin with, the less you save.