For the last several installments of Dissect-O-Stat, we've looked at things on the macroscopic scale. And while drilling a little deeper into exactly what something like the Gini Coefficient is can lend some measure of meaning to the extent of income inequality in the world, it hardly makes it feel personal. Some statistics are just too wide-ranging to ever cozy up to. So today we are going to look at a statistic that is a little closer to home: fuel efficiency.

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Ok, so first, there are two things going on here. One: there is the efficiency with which your engine converts the stored energy in your gas into useful energy in the form of moving engine pistons. And two: there is the efficiency with which the movement of your engine pistons becomes forward progress. Thing one is called thermal efficiency, and is largely governed by engine technology; thing two is called fuel economy and is largely governed by the weight of your vehicle. Thing one has improved dramatically over the last 30 years; thing two has not.

Most people experience both of these factors in one primary way: the gas mileage sticker that's stuck to each car window as you circle the lot. That sticker has a figure for the number of miles one gallon of gas will take you while driving on the typical highway and the number of miles one gallon of gas will take you while driving in the typical city. Highway driving is usually more efficient (hybrids can be an exception to this rule) because (at least in theory) driving on a highway doesn't involve any waiting around, while driving in the city does. Cars are most efficient at around 60 miles per hour, and at that speed, opening your windows decreases your gas mileage more than using your air conditioner does because wind resistance is a really big deal.

The number that appears on your gas mileage sticker gets there through a series of tests. Those tests are performed by the car companies themselves, but the EPA checks up on about 10% to 15% of tests in a given year. The testing works like this: a brand new car is placed on rollers in a lab. The rollers automatically adjust their resistance to accommodate the car's weight and wind resistance at a given speed. A professional driver gets in the car and follows a set routine of speeding up and slowing down so as to simulate the city or highway environment. A computer captures and measures the amount of carbon coming out of the car's tailpipe and extrapolates that to gas used.

According to the Department of Energy, this method of testing is not particularly reality-based: "In the 1980s, an EPA study found that drivers were typically achieving lower fuel economy than predicted by EPA laboratory tests. As a result, EPA required the laboratory-derived city and highway MPG estimates posted on the labels of new vehicles to be adjusted downward by 10 percent for city estimates and by 22 percent for highway estimates to better reflect the MPG real-world drivers can expect." So boo on the EPA for using a bad test, but slight kudos for at least attempting to make up for it.

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In October 1973, the members of the Organization of Arab Petroleum Exporting Countries refused to ship any more oil to countries supporting Israel in the Yom Kippur War (which is to say the United States and most of Europe). At the same time, those countries used their power as a cartel to quadruple oil prices. US President Jimmy Carter famously put on his sweater, rolled up his sleeves, and started pushing various forms of legislation that would reduce America's dependence on foreign oil.

As part of a broad series of environmental legislation (the Clean Water Act, the Clean Air Act and the Endangered Species Act were all passed within a few years of the oil embargo), Congress put in place the Corporate Average Fuel Economy Standards, which require the average mileage across the entire fleet of cars produced by a car company, at minimum, to be 27.5 miles per gallon and the average mileage across the entire fleet of light trucks to be 20.3 miles per gallon.

There are a number of factors that go into measuring the gas mileage of the "corporate average" car or light truck fleet, so let's start to take them apart here. First, the average is sales-weighted, which means that the gas mileage that the Camry gets matters a whole lot more to Toyota than does the mileage that the Yaris gets. Second, the cars and light trucks that count as part of a manufacturer's fleet, for the purposes of CAFE standards, are anything weighing under 8,500 pounds (which means Hummers, as well as Chevy Suburbans and Ford Excursions, are off the hook). Finally, car makers are allowed to use something called Flexible Fuel Vehicles to get gas mileage "bonus points" for certain vehicles. For example, an 18mpg Chevy Suburban that can run on ethanol is credited as if it were a 30mpg gas only vehicle, regardless of what its owner actually puts in the tank (and ps, when was the last time you saw an ethanol station?).

The fuel efficiency requirements under CAFE have increased over time (and will continue to in the years ahead). Car companies that don't meet CAFE standard receive fines (more on that later), though they also earn credits for beating standards by a certain amount in a given year and are allowed to apply those credits to years in which they do not meet the standards. Congress added this provision to accommodate the vagaries of market demand -- it protects car companies in the event of a one year spike in demand on its biggest gas guzzlers.

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When they were put into place in 1973, CAFE standards had an immediate and large effect on the gas mileage of cars traveling America's roadways. Between 1975 and 1985, automakers increased fuel economy more than 60 percent. Overall fuel economy for cars and light trucks peaked at 22.1mpg in 1987. However, since that time, we've been going backward: average fuel economy in 2004 was 20.8 mpg.

And yes, you can blame it on SUVs.

From 1987 to 2004, vehicles increased in size from an average of 3,220 pounds to 4,066 pounds (remeber how vehicle weight governs fuel economy?). For GM, light truck (insert "minivan" and "SUV") sales rose from 28% in 1990 to 56% in 2003. Ford went from 35% to 59%. Overall, light trucks are now 51% of the vehicles we buy, as compared to 30% in 1988.

What's more is that the decrease in fuel economy does not capture the effects of some of the biggest offenders (as we mentioned, Hummers and Expeditions are exempt because they weigh too much), nor does it take into account the token improvements of Flexible Fuel vehicles. In general the Big Six automakers (GM, Toyota, Ford, Honda, DiamlerChrysler, and Nissan), especially the American ones, use Flexible Fuel Vehicles to increase their fleet fuel economy by about 1 mpg.

When car companies don't meet their requirements under CAFE standards, the government fines them $5.50 per .1 mpg under the requirement per car sold. That's a big deal, but not that big. Last year, Ford produced about 3.2 million cars for the US market. If you were to take away the Flexible Fuel Vehicle mileage credit (thereby decreasing fleet fuel economy by about 1 mpg), Ford would find itself with a tax of $55 per car on its hands. That's about $178 million, which, relative to sales of $177 billion, isn't a huge amount. As such, a number of car companies choose to pay the fine rather than meet the standards (BMW, DaimlerChrysler's import fleet, Fiat, Lotus and Porsche failed the automobile CAFE requirement, while BMW and Volkswagen failed to meet the light truck requirement).

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We started this article by deciding to discuss a statistic more immediate and tangible than the broad-based economic indicators that have been featured by Dissect-O-Stat in the past. However, we'll end by broadening things back up. The most important thing to remember about CAFE standards is that they only reflect the average fuel use of a single vehicle. What really matters in the world is the total amount of carbon and smog forming particles (like nitrogen oxides) we are putting in the air. Since 1970, total US vehicle miles have jumped 160%. And so as a nation, even though we have cars that are significantly more efficient today, we produce 64% more carbon by driving around.

Point being: ride your bike.

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