Doing the MPG math

Suppose you had a household with two cars, and each car needs to be driven 10,000 miles per year. One car consumes 34 MPG, and the other car consumes 18 MPG. Since gas is expensive, you want to replace one car. Because of utility constraints, you have two choices:
• Replace the 34 MPG car with a 50 MPG car — a 16 MPG improvement
• Replace the 18 MPG car with a 28 MPG car — a 10 MPG improvement
Which car replacement would save you the most gas? … When you run the numbers, replacing the 34 MPG car with a 50 MPG (a 16 MPG improvement) car saves you 94.1 gallons per 10,000 miles, whereas replacing the 18 MPG car with a 28 MPG (a 10 MPG improvement) car saves you 198.4 gallons per 10,000 miles — more than double the savings. … a textbook case for how common wisdom can fail the common person.

From this blog post (h/t MAKE blog).

It’s an interesting point, and a problem that’s avoided by the metric measurement of gas mileage in L/100km (volume over distance). What we really care about is either (a) how much gas we’re really using for the distance we need to drive, and thus how much money we’re spending on it; or (b) how much particulate matter and noxious gas we’re putting in the air. Both of these are related to the volume of gas used.

Even so, the example above presents something of a false choice. Utility constraints, yes, but replacing an 18 MPG car with a 34 MPG car is still better than replacing an 18 MPG car with a 28 MPG car. Using the right grade gasoline and driving for better mileage also help, no matter what vehicle you drive.