Sorry to jump in guys. The relationship between fuel economy and operating cost is linear. A 3 MPG savings will pay back a $1000 investment in the same amount of miles... no matter where you start from. Using "chuckthetruck"'s notation, the actual mileage calculation is:
M = (N - P) * 1/G * C
example:
(11 - 8)MILES/GALLON * GALLON/$2.95 * $1000 = 1017 miles
(20 - 17)MILES/GALLON * GALLON/$2.95 * $1000 = 1017 miles
The law of diminishing returns comes in when you consider how much it COSTS to increase fuel economy. The first MPG increases are relatively inexpensive compared to what it takes to eek out further savings.
Thanks for the helpful carburetion info "carbking".
P.S. This is my first post to the site. I have learned a lot here and hope to contribute where I can.
