A reader writes:
Small changes in MPG in cars that get poor gas mileage actually have much larger effects than you would think In the example given going from 18 MPG to 20 MPG would be a reduction of 56 gallons of fuel each year if you assume 10,000 miles will be driven. To save the equivalent amount of fuel I'd have to replace my 30 MPG car with one that gets 36 MPG.
The worse the fuel economy of your current car, the more a small improvement actually helps. For example, replacing a 14 MPG with one that gets 18 MPG (something that is very plausible when trading in an old truck or SUV) would be the equivalent of me trading in my 30 MPG car for one that gets just over 57 MPG. This doesn't even factor in the fact that the newer cars are safer and put out less other pollutants as well. This is a case where the low lying fruit is the best stuff to pick, because you get much more bang for your buck.
I wish cars sold labeled the fuel consumption in gallons/10,000 miles. That would make this thing much more visible to the average person. I guarantee that if you asked 100 people which was the better deal going from a car that got 14 MPG to 18 MPG versus one that got 30 MPG to 40 MPG, 90% would pick wrong. But if you said 714 Gallons/10k miles to 556 Gallons/10k miles versus 333 Gallons/10k miles to 250 Gallons/10k miles it's much easier to see.