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Pop quiz: Say you split your driving evenly between a 21 mpg SUV and a 75 mpg Vespa scooter. What would your average miles per gallon be? If you answered 48 mpg you're not alone, as that's what Vespa came up with in their infographic we posted last week (reposted above). The only problem is that that answer is wrong. The real average is 32.8 mpg.The reason for the large difference is that in order to correctly calculate average mpg, you can't simply add both numbers and divide by two. You have t

Imagine, if you will, taking a sheet of paper and cutting it in half. Now take one of those halves and cut it in half again. Now keep repeating the process. As you keep cutting, the difference in the size of the subsequent pieces gets progressively smaller. This simple example is a demonstration of why continuing to increase the fuel mileage of a vehicle has less and less impact once you get beyond about 35-40 mpg.

The cash for clunkers compromise that was reached in Congress this week puts a lot of focus, understandably, on the miles per gallon rating of the cars in question. It's understandable because MPG is how most Americans think about fuel consumption. But, as we've discussed in the past, gallons per mile is another useful metric, and its proponents have updated their site with an article on Cash for Clunkers and the 1 GPM Principle.

Back in June, we took a look at the distinction between miles per gallon (MPG) and gallons per mile (GPM). At first blush, it may seem this is a six of one, half-dozen of another situation. It's not. MPG is the most common number cited when people want to understand how efficient a car is, especially in the US. Richard Larrick, Associate Professor of Management and Organizations at Duke University, says that the MPG number is wholly inadequate to understand the way that car models get cleaner th