Gallons per mile - argument for "low mileage" hybrids?

[smontanaro]

From the mailing list:

    James> I think most people who are interested in the ethics of global
    James> warming find the concept of a Hybrid SOV SUV an oxymoron.  If
    James> someone really wants high fuel efficiency, then they go get it -
    James> in the form of a Prius.

I was listening to All Things Considered on NPR yesterday evening.  They were talking about looking at fuel efficiency in gallons per mile.  Considered that way replacing a 12mpg vehicle with a 15mpg vehicle saves more gas than replacing a 30mpg vehicle with a 40mpg vehicle.  I'm not sure I buy the argument, but the arithmetic looks correct:

    1/12 - 1/15 = 0.01666 gpm savings

    1/30 - 1/40 = 0.00833 gpm savings

This line or reasoning was used to support the argument that we should work hard to get rid of the most fuel-inefficient vehicles (well, duh...) but that you didn't need to replace your Hummer with a Prius to make a big dent in things.  If that reasoning is correct, then having hybrid Ford Escapes and Expeditions makes sense, at least in the short term (5-10 years or so).

Can someone with more experience in this realm comment?

Thx,

Skip Montanaro

[tomsax]

The math is correct, but the argument is tricky and can be misleading.

There was a similar discussion on NPR's Science Friday recently. They talked about expressing fuel efficiency similarly to how they do it in Europe. In Europe they express it as liters per hundred kilometers, so in the US we could do it with gallons per hundred miles. Even better, they suggested gallons per ten thousand miles as that would be in the same order of magnitude as what typical driving is per year, and thus about typical fuel consumption per year.

12 MPG = 833 gallons per 10,000 miles
15 MPG = 667 gallons per 10,000 miles
30 MPG = 333 gallons per 10,000 miles
40 MPG = 250 gallons per 10,000 miles
50 MPG = 200 gallons per 10,000 miles

So switching from 12 MPG to 15 MPG saves 166 gallons per 10,000 miles and replacing 30 MPG with 40 MPG saves 83 gallons. Even going from 30 MPG to 50 MPG only saves 133 gallons per 10,000 miles.

So yes, replacing your mammoth 12 MPG SUV with a merely huge 15 MPG SUV saves more fuel than replacing your already efficient sedan with a more efficient hybrid sedan.

Personally, I find the 12/15 vs 30/50 argument annoying because unless the listener is paying very close attention it makes it sound like you're better off buying a 15 MPG SUV instead of a Prius. That's true, but only in the very special case that you own a 12 MPG gas hog and an efficient sedan, and the SUV at least as much as the sedan.

A much better conclusion to draw from this is that pinheads who drive a mammoth SUV to do what a hybrid sedan can do would make the largest difference in their gas usage by going to the hybrid sedan and saving a whopping 633 gallons (about $3,000) per 10,000 miles.

Of course, if we're going to re-educate the public by putting more information on the EPA sticker, we should also add kwh per 10,000 miles to get everyone thinking about EVs.

[Eletruk]

2009 model cars in California to get greenhouse gas stickers



http://www.autobloggreen.com/2008/06/26/2009-model-cars-in-california-to-get-greenhouse-gas-stickers/