Toyota's new slogan may be "I want my MPG," but some experts are arguing that MPG is a backwards way of measuring a vehicle's fuel efficiency. Instead, they say, we need GPM.

We Americans are no strangers to weird measurements. In metric-friendly countries, water freezes at 0 and boils at 100. In America, there are twelve inches in a foot and 5280 feet in a mile; breakdowns that are a little tough to wrap your head around compared to the powers-of-ten-tastic metric system. Our MPGs are an equally confusing measure. Treehugger recently blogged that the relationship between the amount of gas consumed by a vehicle and its MPG rating isn't linear ... it's curvilinear. I suck at math, but a quick look at this graph made the distinction fairly clear:

In other words, the gasoline savings of replacing a 15 MPG car with a 20 MPG car is about equal to the savings of switching from a 30 MPG car to a 60 MPG car.

Here's a quick breakdown of what gallons-per-mile looks like:

15 mpg = 660 gallons per 10,000 miles
20 mpg = 500 gallons per 10,000 miles
30 mpg = 330 gallons per 10,000 miles
45 mpg = 220 gallons per 10,000 miles
60 mpg = 160 gallons per 10,000 miles

Such a system may not make it immediately obvious how much a week's worth of gas is going to cost you, but it makes vehicles' relative fuel efficiencies much clearer. So despite the current focus on making small vehicles even more efficient (for instance, Honda coming out with an only-slightly-more-efficient hybrid version of the Civic), the biggest across-the-board gains will come from taking the least efficient vehicles off the roads. To wit: replacing the 2008 Ford Expedition, which gets just 12 MPG in the city, with something that gets just two miles per gallon more represents the same relative gas savings as replacing a 28 MPG car with a 40 MPG car. As Ecogeek points out, "this is why the Chevy Tahoe hybrid won green car of the year this year."