Thursday, January 31, 2008

Gallons per Mile?

Here’s something quick and easy the government could do. The Europeans measure gas mileage in liters per 100 kilometers, which makes a lot of sense. After all, when we set out on a journey we do not decide how much gas to buy and then determine how far to go; the constant is the journey and the variable is the amount of gas we need.

Changing to gallons per 100 miles – I am not bold enough to suggest we go metric! – could also have a serious impact on our perception of fuel economy and thereby on our carbon footprint. The reason is that our brains don’t process reciprocals well. Consider this table comparing the two measures (I have rounded a little in a couple of cases):

12.5 mpg = 8 gallons per 100 miles
14 = 7 gallons per 100 miles
17 = 6 gallons per 100 miles
20 = 5 gallons per 100 miles
25 = 4 gallons per 100 miles
33 = 3 gallons per 100 miles
50 = 2 gallons per 100 miles

Note that replacing a 12.5 mpg gas guzzler with a less profligate17 mpg vehicle saves twice as much as replacing a 33 mpg compact with a 50 mpg hybrid. Not that I am against high-efficiency vehicles, but I wonder whether the emphasis should be at the other end of the scale. If we could get everyone up to say 25 mpg we would be doing an enormous service. Recent legislation to change CAFE standards to an average of 35 mpg will of course help.

That little word “average” invites caution though; you need to average the reciprocals, which is exactly what CAFE does. So, in a way the EPA already uses gallons per mile for its calculations. They just don’t want to tell you. Why not change the way we display it on the new car sticker?

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