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Today’s Note: Cars, Big Butts and MPG

January 14, 2012

Apparently, people getting fatter is holding back fuel efficiency progress among US automakers’ fleet mpg averages. Really. It couldn’t have anything to do with all the extra traffic controls being constantly installed in our most populated areas. Surely not. Don’t even suggest that construction zones play a role. It’s big cars and big butts. According to the ExtremeTech.com article titled “Why real-world mpg isn’t improving: Big cars, big butts smother automakers’ efforts,” the fault is ours:

Technology aimed at making cars more efficient has resulted in modest mileage gains in the cars we’re driving. Are the automakers and Big Oil fibbing about their efforts? More likely the fault lies with us: Given the choice, we buy bigger cars and SUVs with larger engines, in no small part because we’re 25 pounds (11kg) heavier than four generations ago.

[all italics are mine, and are intended to emphasize quotes from the article]

The piece, written by Bill Howard, is a reasonably complete and well-considered assessment of the current state of fleet fuel economy among major automakers. Don’t let the absence of real science dissuade you from reading it.

According to Howard, “Fuel economy increased 60% from 1980 through 2006, according to MIT economist Christopher Knittel, “all things being equal.” But they weren’t equal. Over the same 26-year period, the average economy of vehicles sold in the US went up just 15%, probably because curb weight went up 26% and horsepower more than doubled (up 107%).

There is absolutely no doubt this has contributed greatly to constraining fuel economy advances.  The lack of progress is manifest. Howard makes his case compellingly:

If we had the same size cars and engines as in 1980, the US fleet average fuel economy would have increased from 23 mpg in 1980 to 37 mpg. Instead, the nation’s 225 million cars and light trucks are getting about 27 mpg. That’s despite the advent of hybrid vehicles, higher-efficiency engines, fuel injection, turbocharging that allows for four-cylinder engines with the power of a six-cylinder, sophisticated camshafts controlling engine-valve opening and closing, low-rolling-resistance tires, transmissions with as many as eight speeds, and more aerodynamic body shapes.

At the end of the day, something is missing from all the analysis, though. Somebody needs to account for changing driving conditions – more stoplights, more stop signs, slower speed zones, more traffic, and more construction – when assigning blame for fuel efficiency. On my average commute, I spend at least twice as much time braking and idling as I did 20 years ago. And a typical long-distance trip in the summer involves an astonishing sum of time sitting idling in or crawling through construction zones. Before we assign responsibility for automaker’s failures to get us 50mpg, let’s see somebody measure the drag on fuel economy introduced by those conditions.

Howard’s article is good, but I’d like to see harder questions asked than copping out to the “fast cars and donuts steal our fuel economy” theory.

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