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What's in the Autoclave? A Blog about Science

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Miles per gallon, versus gallons per mile [Jul. 24th, 2008|12:50 pm]
What's in the Autoclave? A Blog about Science
There's a very cool article in the June 20 issue of Science (I'm a bit behind in my reading) that talks about a better way of thinking of fuel efficiency in vehicles. (The article is here for those with access to Science online.)

One of the few things I remember from my undergraduate statistics class is the importance of choosing the right thing to measure. You can make a statistic out of any measurement(s) you like, but it is important to step back and think about whether you are really measuring what you want to, whether the comparisons you make are actually the most appropriate for a given problem. The Science article argues that the standard United States measurement of a vehicle's fuel efficiency -- the number of miles it can travel per gallon of gasoline -- may not be the most appropriate measure when thinking about that vehicle's environmental impact.

Miles per gallon (mpg), as the authors point out, is a great measure of a vehicle's useful range -- how far can you travel without refueling? And so for a consumer purchasing a car, it is a useful metric. However, an environmentally-conscious consumer -- or a policymaker setting fuel efficiency standards -- might be more interested in the reciprocal of this number, the number of gallons of fuel that it takes to travel one mile. (Or, since this is likely to be a decimal number with one or more zeroes in front of it, the number of gallons it takes to travel 100 miles, or 1000, or any other arbitrary distance.)

Why does this reshape the way we think about fuel economy? Well, most drivers (apart from vacation travel) use their vehicles to travel a set distance every day -- their commute to work. In most cases, the distance part of miles per gallon (distance divided by amount of fuel consumed) is effectively a constant. The variable is the number of gallons which the car consumes to travel that fixed distance. So consider a hypothetical driver whose vehicle gets 20 mpg (or, reciprocally, uses 0.05 gallons to travel one mile.) He or she is thinking of buying a new car, and his choices are one that gets 25 mpg versus one that gets 15 mpg. The 25 mpg car uses 0.04 gallons per mile (an improvement of 0.01 gallons per mile), while the 15 mpg car uses 0.0667 gallons for the same distance (a loss of 0.0167 gallons per mile.) A difference of 5 mpg in fuel economy is not the same at the lower end of the scale as it is at the higher end.

The authors point out, then, that an improvement in fuel efficiency from 16 mpg to 20 mpg actually has more of an effect on gas consumption than a change from 34 mpg to 50 mpg -- a savings of 125 gallons per 10,000 miles in the first case, compared to only 94.1 gallons per 10,000 miles in the second. They argue that stating a vehicle's fuel efficiency in terms of gallons per mile (or similar) would help consumers make more environmentally-conscious (and economically-conscious) decisions. People, including policymakers and environmental advocates, may undervalue the importance of small improvements at the lower end of the mpg scale, focusing instead on the high end of the mpg scale, because it seems counterintuitive that a small improvement in a low mpg vehicle should have more of an effect on fuel consumption than the same absolute mpg difference at the higher end of the scale.

Will this stop me from being disdainful of "hybrid" SUVs? No. The more environmentally friendly choice remains a high mpg subcompact. But averaged over the useful life of the vehicle, every little drop of fuel counts.