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."