Consumer Reports on diminishing returns, the backwardness of our mpg system vs. Europe's "gpm" (liters/100 km) system, or "how Cash for Clunkers perhaps wasn't such a bad idea after all":
http://news.consumerreports.org/cars/2011/08/buyers-are-trading-toyota-priuses-for-chevrolet-volts-heres-what-would-make-a-bigger-difference.html
http://news.consumerreports.org/cars/2011/08/buyers-are-trading-toyota-priuses-for-chevrolet-volts-heres-what-would-make-a-bigger-difference.html
In our tests, the Prius gets 44 mpg overall, and after a lot of number crunching we have calculated that Volt owners will be getting the equivalent of 58 mpg (counting electric energy, and assuming 70 percent electric driving). At that rate, in the course of 12,000 miles, roughly a year’s use, a typical Volt will use about 66 fewer gallons than a Prius.
What if GM pickup buyers switched en masse to [GM's] hybrid [pickup truck]? In a year, each [19 mpg] hybrid Silverado would save almost 226 gallons over a traditional [14 mpg] truck.
Update: President Obama announces fuel-economy standards for big trucks (as in medium and heavy duty trucks), directly relevant to the above: http://news.consumerreports.org/cars/2011/08/new-fuel-economy-standards-for-big-rigs-fire-engines-and-work-trucks.htmlThe same principle of diminishing returns holds true even when we use more realistic examples. Switching, say, from a 30-mpg Honda Civic to a 44-mpg Prius saves 127 gallons per year. That’s great, but still 99 fewer than the 226 realized from goosing a gas guzzler from 14 to 19 mpg.
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