The Union of Concerned Scientists points out that switching from an S.U.V. that gets 14 miles per gallon to one that gets 16 would save the same amount of fuel as swapping a 35-mile-a-gallon car for a 51-m.p.g. new generation gas-sipper. This is not an argument for more S.U.V.’s. It simply shows that we can wring savings from modest efficiency gains in products we already use.

Since this is somewhat counter-intuitive, I thought a brief explanation might be useful.

The confusion stems directly from the units used to express fuel economy in America. “Miles per gallon,” while perhaps clearer for consumers, is a difficult framework in which to think about resource consumption (since the resource – gallon of gas – is, by definition, fixed).

On the other hand, consider the alternative — flip the term and talk about gallons of fuel used per mile traveled. In the example above, an SUV that gets 14 mpg burns 0.0714 gallons of fuel per mile traveled, or, to make things easier, 7.14 gallons of fuel per 100 miles traveled. The car that gets 35 mpg burns 2.86 gallons of fuel per 100 miles traveled.

First of all, from the perspective of limiting absolute energy consumption, the car is clearly a better choice, and that fact should not be lost in this discussion.

But consider the impact stemming from relative improvements to the two vehicles. If I improve the SUV’s fuel economy from 14 to 16 mpg, my fuel consumption per 100 miles traveled drops by almost a gallon – from 7.14 to 6.25. In other words, I have saved one gallon of fuel as compared with my baseline scenario. To get the same improvement from the car’s baseline, I have to decrease my fuel consumption from 2.86 gallons per 100 miles traveled to 1.96, corresponding to a fuel economy improvement to 51 mpg:

The optimal scenario would of course be to upgrade all your vehicles to 51 mpg (or more). However, given that such an ideal is often not practical or reasonable, comparative analysis as described here can be very beneficial from political, business, and personal perspectives.
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Update 1/20/09:
The Energy Analysis blog has a similar post about this issue, and includes the following relevant graph showing the non-linear relationship between fuel consumption and mpg:

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If you want to get quickly up to speed on how the US alternative fuel vehicle market is shaping up over the next few years, I recommend the following two posts:

“Everything you could want to know about the plug-in hybrid and electric vehicle announcements at the Detroit auto show” (from Climate Progress via Calcars.org; note the excellent “ways to stay informed” links at the bottom);

“Green Cars of 2008: Mega-Ginormous Summary of the Year” (from Treehugger).

Image: Treehugger

First, I’d like to post highlights from presentations I heard from Lew Fulton and Pierpaulo Cazzola of the IEA, based on energy demand projection research that went into the World Energy Outlook and Energy Technology Perspectives. (Side note: the just-released WEO 2008 Executive Summary is a must read.)

To start with, here are a couple of great slides from Mr. Fulton’s presentation, “Transport, Energy, and CO2 in Asia: Where are We Going and How Do We Change It?“:

The above slide shows IEA projections of global car stock by region (y-axis is in millions). Note the exploding dominance of China, especially after 2015-2020. This graph highlights both the incredible challenge we face to limit the energy and environmental impacts of vehicles worldwide, as well as the critical importance of guiding the inevitable growth of vehicles in the developing world in as sustainable a direction as possible.

On energy consumption and greenhouse gas emissions, Mr. Fulton proposed that it is both economically and technologically reasonable to target a 50% reduction in global light duty vehicle energy intensity by 2030. This loosely means reducing average car energy consumption from approximately 8 to 4 l/100-km. Note that some vehicles, such as the Prius, already achieve a fuel economy in this range.

However, current policies are not even close to guiding the vehicle fleet to this target. The following slide shows baseline vehicle fuel economy projections to 2050, taking into account all current legislation:

There is clearly a gap between what legislators (and the vehicle industry) are targeting, and what is currently possible (and required to meet necessary global GHG reductions).

Looking out to 2050, the IEA proposes that the majority of CO2 emissions savings from the transportation sector will come from improvements to conventional gasoline and diesel engines and traditional hybridization. This was surprising to me, as I expected electric vehicles and plug-in hybrids to play a bigger role. Additional savings are projected to come from some combination of electric vehicles and fuel cell vehicles, but the extent of those savings will depend on future technology improvements:

Fortunately, in the case of conventional engine improvements and traditional hybridization, pricing (in theory) shouldn’t be the issue, as the fuel savings are on par with the additional technology cost for these vehicles:

Lastly, I’m posting up the conclusion slide from a talk with some overlapping content given by Mr. Cazzola called, “Fuel Economy as a Means to Avoid Future Greenhouse Gas Emissions from Transport“:

The conclusion about monitoring increases in weight and power is critical, and something I will address in another post.