Diesel vs. Unleaded: The Truth About Fuel Economy

Ahh, you remember it, don't you? Sitting in the back of your family's station wagon as a kid, your father pulls up to the gas pump and a station attendant—an occupation about as extinct as the dinosaurs—came running out with a rag in his hand, leaned into the driver's window and said… "Diesel or unleaded?"

Those days are long gone. And I'm not just referring to the attendant. Nowadays, at least in the U.S., you can expect to find three things at a gas pump: self-serve, pay first, and unleaded gasoline only.


Misconceptions about diesel fuel being "dirtier" than unleaded gas several years ago ultimately led to diesel's obsolescence in the more environmentally progressive United States. Car companies began to manufacture cars that, for the most part, ran on traditional gasoline.

In response, however, steps were taken to improve the cleanliness of diesel and now major car companies have begun to sell diesel-powered vehicles to consumers once again.

In the past, especially in the United States, diesel fuel was considerably dirtier than unleaded gasoline. Vehicles in the United States also tended to be built for traditional gasoline, with the exception of large trucks, buses, and semis. In recent years, however, steps have been taken to improve the cleanliness of diesel fuel through regulation, and many companies have begun selling popular consumer cars that run on diesel fuel.

One such company is Isuzu Motors, known for its popular 5-passenger Ascender SUV, and two models of high-performing pick up trucks (the i-290 and i-370). Isuzu's diesel engines have won a laundry list of awards in past years including "Truck of the Year" by Motor Trend Magazine, "Executive Diesel Car of the Year 2002" by Diesel Car Magazine (a publication in the U.K.), "Germany's best small and compact car of the Year 2001" by Mot Magazine (published in Germany), as well as having been named multiple times by Ward's Communications of the U.S. as one of the "10 Best Engines." In addition, Isuzu vehicles were voted the top medium-duty truck brand six times in seven years by truck dealers. Impressive, but not fully convinced that diesel's better? Keep reading…

Diesel fuel actually burns more efficiently than gasoline, thus having a better fuel economy—about 30% more efficient on average. Why?

Diesel is denser than its unleaded competitor. Proponents of gasoline are quick to point out that diesel engines produce more greenhouse emissions, which is true, emissions with diesel are about 15% higher than those of traditional unleaded gas due to higher volumetric energy density. However, the increased fuel efficiency more than offsets the higher percentage, so in the long run, diesels produce less emissions— 15% more per gallon used, but a trip in the good ole family car will use 30% less fuel to get to wherever you're going.

Granted, this wasn't the case with Dad's old wagon. Back then, before the anti-diesel crusade, diesel engines were creating more emissions because they simply weren't efficient. Now, with the modern diesel engine achieving a 20- 40% better fuel economy, it's time to give diesel a second look.

But that's not the only exciting news about diesel. Originally, the diesel engine was created as a way to run vehicles using vegetable oils. While farmers originally couldn't compete with big oil companies, with gas prices soaring, there's been a renewed interest in alternative fuel sources—and food-based diesel fuels can be produced quickly, cheaply, and with the abundance of products grown in our very own country. These "hybrid diesels" also known as "biodiesel" are compatible with existing diesel engines and can also be made from waste oils gathered from restaurants or homes—an alternative that serves several ecological benefits at the same time.

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