I wrote a couple of provocative blogposts last week (including one that went careening around the Internets and became my all-time most popular post). They elicited reactions ranging from the mildly inquisitive to the genuinely spittle-flecked. Good times!

But while I always welcome an argument—even a heated one—I was chagrined to learn that many readers misunderstood my intended point. That’s usually the author’s fault, as it was this time, and my post titles didn’t help matters.

So, I’m going to clarify a couple of things now:

  • While it’s true that the biggest fuel savings by far can be found at the bottom end of the fleet, I don’t think that 18 mpg minimum is an acceptable stopping point for conservation. We should aim for much higher fleet averages.
  • While it’s true that the there’s a bigger fuel consumption difference between 15 and 18 mpg than there is between 50 and 100 mpg, there is, of course, an even bigger difference yet between 15 and 50 (or 15 and 100). It’d be terrific if 100 mpg cars became commonplace.
  • On the other hand, it’d be even more terrific, perhaps, if cars just became less commonplace than they are now. I didn’t say this earlier, but I should now: from the perspective of fuel conservation, we can also do other things besides vehicle efficiency standards. I think it’s of paramount importance to reduce driving too.

  • All that said, the surprising math of mpg clearly shows that once we get above 50 mpg or so, the fuel conservation returns diminish very substantially. That much is not debatable.

    On the other hand, my interpretation of that mathetical fact is debatable. Here’s what I think: super-efficient future cars would be nice, but our climate’s future does not depend on big automotive technological breakthroughs, or on giant R&D investments. We can do it now.

    I don’t know about you, but I take the mpg math to be tremendously good news. The biggest reductions in fuel consumption—and the biggest benefits to the climate—come from doing stuff we already know how to do. We should make conventional low-mpg cars more efficient. And we should support lots of alternatives to carbon-intensive transport.

    [Updated 5:20] The bad news, on the other hand, is that we must actually do things now to support carbon alternatives. We’re not inventing our way out of this fix. And anyway, the perpetually right-around-the-corner vehicle breakthrough probably wouldn’t really accomplish very much—at least not compared to just making our existing SUVs a little more efficient. So, we have to roll up our sleeves, redesign our communities and lifestyles, and meet our climate goals with the tools we have right now.

    Finally, here’s a tidbit to get comments started:

    You save more fuel switching from a 13 to 17 mpg car than switching from a 50 to 500 mpg car.

    Given that, where do you think our priorities should be?

    Math and charts are below…

    Miles-per-gallon math is pretty counterintuitive. You can find an explanation here and you can check my math (and do some of your own) here (xls). For some people, this chart helps:

    mpg curve_324

    [Update 12/27] And for commenter barry, here’s a chart showing the same information, but this time scaling from 1 to 100 mpg. This really sharpens the importance of low-efficiency vehicles:

    mpg_328