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January 21, 2005

NEW Math

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.

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.

Posted by ClarkWD | Permalink


Sorry Clark, I must disagree. SUVs have other social costs that outweigh any gain in efficiency made by a hybrid SUV.

Posted by: Rob Harrison AIA | Jan 21, 2005 1:18:03 PM

Your new math depends on an assumption; that it is just as cheap & easy to hybridize a SUV as it is for a car.

For example, I would expect the size of the battery would be proportional to the power of the vehicle. Since SUV's are heavier, they'll need more power and thus larger batteries than needed for a lighter vehicle.

It's probably similar for all the other parts of the drivetrain, they will all need to be bigger, and therefore more expensive than for cars.

It is true that you'll save more gas with a hybrid SUV, but if it costs more to save that gas, there is no clear advantage to hybrid SUV's.

Posted by: jimsum | Jan 21, 2005 2:23:54 PM

Rob, while I agree with you that SUVs do have significant social costs aside from their fuel efficiency, it should be noted that the Escape mitigates some of these as well.

One major cost of SUVs is the extra danger they pose to other cars and their occupants. Their size is often the factor implicated, but their construction is also responsible. In particular, SUVs are framed very differently from cars. In a car the vehicle's body, floor pan and chassis form a single rigid structure, referred to as unitized construction. On the other hand, SUVs are constructed by bolting an independently constructed body onto a truck chassis. In a collision, this connection often severs, causing the truck's body to keep moving forward (inertia and all). If the collision is with a smaller and shorter car, the resulting continuation often takes the (heavy) body over the car's protective cage and into the passenger compartment via the windows. This is a primary reason why SUV crashes result in so many deaths and injuries for occupants of the other vehicle involved.

However, the Escape (and, for that matter, many of the small SUVs), are not made this way, but rather through unitized construction. As a result they are significantly safer to other drivers, with correspondingly lower social costs. Furthermore, unitized construction is lighter, and a Ford Escape Hybrid is a full 2500 lbs lighter than their popular Expedition model (7300 to 4720, gross weight, comparing 4x4 models). Weight is also a big factor in social safety costs; when big cars hit little cars, the little ones lose.

No, I'm not a Ford rep, and I do agree with you that having every SUV owner switch to a Prius (or even to an Accord wagon) tomorrow would have a lot of social and environmental benefits. But the bottom line is that if your friend owns a 1996 Explorer and loves SUVs, talking them into trading it in for an Escape will make a huge difference, both for the planet and for their fellow drivers.

Posted by: Matt | Jan 21, 2005 3:45:20 PM

JimSum - The hybrid Escape has an EPA-estimated fuel efficiency of 36 city/31 highway. You're right, the large size of the vehicle reduces the benefit of the hybrid technology. But it's still a fuel efficiency upgrade from an ordinary SUV -- and a heck of a lot better than my family's Outback.

And just to be clear -- I'd rather that SUV drivers switch to the Prius, or something with even better fuel economy, than to the Escape. But there are a lot of people out there who are sold on SUVs, and I can't do anything about that. For them, switching from a Hummer to an Escape is a pretty good thing -- and, in fact, brings more overall fuel savings than switching from, say, a Corolla or Camry to a Prius.

Posted by: Clark Williams-Derry | Jan 21, 2005 4:29:50 PM

Clark, unitized construction doesn't mitigate the increased danger (and thus social cost) SUVs and LTVs pose to pedestrians, bicyclists and motorcyclists because of their tall flat fronts. According to a recent article in Motorcycle Consumer News, compared to two-vehicle accidents with cars, motorcyclists are 2.4 times more likely to die in a collision with a SUV/LTV, and injuries are more severe in non-fatal accidents. I believe the statistics for pedestrians are similar. Cars, with their lower, more aerodynamic front ends, tend to scoop pedestrians, bicyclists and motorcyclists up rather than mowing them down.

If you assume (as I do) that fewer people walk and ride bikes, scooters and motorbikes because of the threat of injury, then SUVs are negatively affecting that indicator as well.

Posted by: Rob Harrison AIA | Jan 22, 2005 11:44:13 AM

Matt, I agree that hybrid SUV's are better than regular SUV's. All vehicles should be hybridized because, as you point out; it is easier to convince a SUV owner to buy a hybrid SUV than a Prius (even though I think most SUV owners don't need any more capability than the Prius offers :-)

I was responding specifically to the argument in this article; that the benefits of making hybrid SUV's are greater than those from making hybrid small cars. This is true, but I also think it will likely cost more hybridize SUV's; so it isn't obvious whether hybridizing SUV's or small cars is more beneficial, when costs are taken into account.

Posted by: jimsum | Jan 24, 2005 8:18:45 AM

Did you know?

76 years ago the President of General Motors predicted 80-mpg by 1939; 69 years ago Ford Motor Co. tested a 170-mpg Pogue carburetor; 32 years ago Shell Oil Co demonstrated a 376-mpg automobile; 28 years ago a 100-mpg Ford V-8 was demonstrated; 22 years ago Peugeot advertised a 72-mpg @ 56-mph Diesel. 3 years ago an English newspaper article announced a 104-mpg Toyota Diesel and 94-mpg VW/Audi Diesels. Commercial fuel cell vehicles have been available in Europe for years. Many U.S. Patents exist for devices that separate the elements of water for use as fuel, one patent #1,380,183 was granted 84 years ago.

The other side is a composite of 8.5”X11” documentation for four of the above statements. If you wonder why this technology is not available to you see: byronw.www1host.com.

Byron Wine
([email protected])
Manassas, VA

(I distribute this two-sided page of information because no local politician or media will inform the public of decades old energy technology

This format prevents attachments. My site has over 100 documents.

Posted by: Byron Wine | Mar 5, 2005 11:01:18 AM