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The mpg confusion

Filed under: — gavin @ 23 September 2008 - (Español) (Français)

What reduces emissions more?
A. Someone swapping their old SUV (which gets 12 miles per gallon) for a hybrid version (18 mpg) or
B. someone upgrading their 25 mpg compact to a new 46 mpg Prius?
(ignore for a minute manufacturing issues or driving habits and assume the miles driven are the same).

The surprising answer (for those who don’t work it out) is A. It’s easy enough to see why this is the case. If the driving distance is 100 miles, then for case A the saving in fuel used (and hence emissions) is 100/12-100/18 = 2.8 gallons, while for B, you have 100/25-100/46 = 1.8 gallons. The confusion arises because people like to think linearly about numbers, not inversely, and so tend to assume that a similar change in mpg has a similar impact on fuel usage. This is not however the case – improvements in efficiency at the low end of the scale are much more useful at reducing emissions. This is actually a very general point – when trying to raise efficiency it is always sensible to start with the least efficient processes.

This confusion got some attention a couple of months ago after a piece that was published in Science by Larrick and Soll. They tested peoples instinctive reactions to changes in mpg numbers and found that people very often got it wrong, leading to less than optimal decisions. They also tested a different way of giving fuel usage information (the number of gallons used per mile), and since this is linear in emissions, people made the correct judgment much more often (it’s worth noting that the standard in most of Europe is already litres per 100 km). Rewritten in those terms, the choices above become:

A. Someone swapping their old SUV (which takes 8.3 gallons to go 100 miles) for a hybrid version (5.6 gallons/100 miles) or
B. someone upgrading their 4 gallons/100 miles compact to a new 2.2 gallons/100 mile Prius?

Much easier, right? The authors of the Science piece are trying hard to get US manufacturers and the EPA to switch over from mpg to this new standard (though they prefer gallons/10,000 miles). It all seems eminently sensible to us.

356 Responses to “The mpg confusion”

  1. 351

    re: 349:

    Very interesting. It’s always good to see practical steps taken. I do wish they would explain the process a bit better, as they make it sound as if the process itself does not result in GHG emissions, which makes no sense. (What is really meant is that the process ends up displacing fossil-fuel generated GHG emissions, for a net improvement.)

    For more detail, see:

    (Captcha says “not grateful,” which is the inverse of my feeling.)

    Funnily enough, that was an “incorrect Captcha,” though I don’t see how. . .

  2. 352
    Marcus says:

    Re: 350: Sekerob: On what timescale does corn-based ethanol have 3x the carbon footprint of gasoline? Do you have a reference? I’m guessing you are looking at a short time scale analysis of the land-use change carbon emission papers, but my impression was that on a long enough time scale (50 years or so) that switching to corn ethanol would eventually reduce carbon emissions. Mind you, I still think corn ethanol is a bad idea, but it is good to be precise.

    Re: 348: Floyd: My impression is that the cellulosic parts of sugarcane ethanol production are referred to as “bagasse” and are often burned to run the fermenters, and therefore are a part of what makes ethanol from sugarcane a more efficient process than ethanol from corn. I was also under the impression that even with transport costs factored in, Brazilian sugar cane ethanol still beat US corn ethanol in the US in terms of carbon footprint, but I don’t have a good citation for that.

  3. 353


    Right, the “bagasse” is what the cane farmers of NSW formerly burnt off in the fields, and will now be used to generate electricity.

  4. 354
    Hank Roberts says:

    Analogous to the “miles per gallon” improvement — simple highly efficient woodburning stoves are coming into use worldwide. Here’s one:

    In the last 13 years, variations of the Rocket Stove have been built in over 20 countries.

    · Efficiency: 12-42%. The efficiency depends on type of a heat exchanger used.
    · Construction: Simple to construct with a number of different materials. The simplest Rocket Stove can be built with thick tin cans and wood ash (5,000 of these were built in refugee camps in Zaire).
    · Material costs: $0-$20 US. In Honduras we made a simple refugee version of this stove for approximately $1.50 US in material costs.
    · Life expectancy: Is 2 weeks to ten years depending on the materials used ….

  5. 355
    Sherlock says:

    Carbon neutral insurance companies like ibuyeco offset vehicle emissions by calculating engine size, fuel type and mileage. What else could be taken into consideration, shouldn’t the age of the vehicle apply?

  6. 356
    Charles says:

    Or even better, one could swap his SUV for a hybrid.