In yesterday’s post on car choices, I mentioned something that at least one reader found counterintuitive:  that increasing automobile efficiency has diminishing returns.  All else being equal, switching from a 15-mpg SUV to a 30-mpg car is twice as beneficial as switching from a 30 mpg car to a gas-sipping, 60-mpg hybrid.

Here’s why.  Let’s say you’re taking a trip that’s 60 miles long.  The SUV burns 4 gallons of gas  (60/15=4).  The car burns 2 gallons—saving 2 gallons vs. the SUV.  The hybrid burns one gallon—saving 1 gallon vs. the car. Clearly, if you have the option of upgrading an SUV to an ordinary car, or upgrading a car to a hybrid, the former is the better choice:  it saves twice as much gas.

In fact, if you do the math—and from an emissions standpoint alone—it’s just as important to switch someone from a 15-mpg car to a 30-mpg car as it is to convince someone with a 30-mpg car to stop driving altogether.  For a 60 mile trip taken (or avoided) it’s still 2 gallons of gas saved.

This, I hope, is clear enough. But all sorts of depressing things follow as consequences of the math.

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• Here’s one.  Imagine two families: one trades in their 30 mpg car for an SUV, the other upgrades their 30 mpg car for a hybrid.  You might think that the average gas mileage of the two families went up, from (30mpg + 30 mpg)/2 = 30 mpg to (15mpg + 60mpg)/2 = 37.5 mpg—a net boon for the environment.

But you’d be wrong. The average gas mileage of the two families actually declined by 20 percent, from 30 mpg to 24 mpg.  Here’s an example to prove the point:  driving the two 30-mpg cars 60 miles each burns 4 gallons of gas total, or 2 gallons for each car.  Driving the hybrid and the SUV 60 miles each, however, burns 5 gallons of gas total:  four for the SUV, 1 for the hybrid.  Average mpg went down, not up.

As it turns out, for every family that trades a car for an SUV, two families have to upgrade to hybrids just to keep the average fuel economy of the vehicle fleet constant.  This is just a hypothetical example, but it’s true in real life too:  even though hybrids are flying off the lots, they’ll have to fly off faster still to make any appreciable progress against the rising tide of SUVs.

Of course, the math also suggests a solution—one that I’m not particularly fond of in the abstract, but can see the wisdom of in the concrete.  Absent market conditions or other forces that could slow down the vehicle upsizing trend, it seems that hybrid SUVs (such as the Ford Escape) may hold as much or more promise for improving the overall fuel efficiency of the vehicle fleet than does the super-efficient Prius. I’d much rather drive a Prius. But if the Escape is able to get people out of their 15 mpg behemoth into a behemoth that can get 36 mpg in city driving, I have no objections.