Diesel Fuel Vs. Unleaded Gas: Which Is Cheaper?


As you drive by the gas station, you’ve probably noticed that, underneath all the prices (below even the premium you almost never buy), there’s diesel, and it’s always expensive. You probably think to yourself: “Man, am I glad I don’t pay that per gallon.”

But you might want to. We did some number crunching and it turns out that a diesel might be the way to go to get more mileage out of every gallon.

But how much do you really save? And what is diesel fuel, anyway?

Diesel: The Surprisingly-Efficient Fuel

Diesel was invented back in 1892, courtesy of Rudolph Diesel, a German inventor and engineer who was trying to figure out how to turn coal dust into fuel and develop something more efficient than the then-popular steam engines. In fact, a diesel engine technically has nothing to do with diesel fuel: Diesel himself used to run demonstrations with his engine burning, of all things, peanut oil.

The efficiency in diesel engines comes from two places; the design of the engine and the way diesel fuel, also called petrodiesel, is refined.

Gasoline engines, as we all know, use spark plugs to trigger internal combustion. Diesel doesn’t use spark plugs. Instead, the cylinder compresses the air to very high pressure and temperature, and then the diesel is injected straight into the cylinder. The combustion is triggered by the fuel entering that high pressure and high heat environment, pushing the cylinder, which then presses back, and the cycle starts over again.

A diesel engine is also called a “lean burn” engine: that is, it uses more air than needed in the fuel-to-air mixture in order to trigger a reaction, unlike gasoline engines, which use the exact mix needed to trigger combustion. That means the engine uses less fuel.

Also missing from a diesel engine: a throttle. With no throttle, no energy is lost, and it can all go to driving the engine. This also makes it a lot harder to stall a diesel engine, and gives it more torque.

Similarly, diesel fuel has more energy density than gasoline: 11% more, according to tests run by European governments. So when you burn diesel, you not only get more energy than gasoline, more of that energy also goes to the motor.

So, why aren’t we all driving diesels?

There’s Always a Catch

A few reasons. The first, and biggest, is that diesel engines have a possibility of becoming “runaways.” As long as fuel is provided to the engine, it’s just going to keep going; that efficiency makes them very self-sufficient. Needless to say, this safety problem has been reduced throughout the years, but back in the early 20th century, when we were fighting over what fuels to use and people were actually afraid of cars, the “runaway” problem was a serious one.

At the time, gasoline was also much cheaper than diesel; even with fuel prices rising, it still is, although less so now. Worst of all, diesel fuel can actually become a jelly at temperatures as low as five degrees Fahrenheit. It needs chemical additives to stay liquid.

Another nail in the coffin, and a price factor today, is that diesel and heating oil are made in pretty much the same way. So just as during the summer, gas prices get a huge bump, in the winter, diesel prices rise.

And finally, there was the sulfur problem: diesel, back in the early 1900s, was full of sulfur, and when you burned it, it stank.

The irony is that diesel fuel is actually safer than gasoline in an important respect. Gasoline doesn’t burn nearly as easily as the movies would have you believe, but diesel has an even lower flammability. This is why pretty much every heavy-duty vehicle, from big rigs to battleships, runs on diesel: the efficiency you get is worth the cost to governments and corporations, and the safety aspect can’t be overstated.


Why Does Diesel Cost So Much Today?

Ironically, it may not: it really depends on demand in your area. Farm country tends to see lower prices because there’s more demand.

Still, the price has risen over time. Just like gasoline-refining techniques have changed over time to make them more efficient and less polluting, so have diesel-refining techniques, and those techniques aren’t free.

All diesel today is Ultra Low Sulfur Diesel, or ULSD, which pollutes less (and smells better), but costs more to make. Also, since demand for diesel isn’t nearly that of gasoline, less refining capacity is dedicated to it, and even for gasoline, refining capacity is at a premium. Then there’s transportation and marketing costs, which boost the price even more.

Nonetheless, it is more efficient that gas: that’s not in dispute.

But If I Did Switch to Diesel, What Do I Get for My Money?

That’s where the math comes in.

We took three popular diesel vehicles that also have gasoline versions: the Volkswagen Passat and Golf, and the Audi A3. We figured out their average total mileage per gallon and the capacity of their gas tanks, and then calculated:

Then we worked out the cost per mile…and how much more you’d pay, per tank, to get the same mileage out of a gasoline-powered car.

The charts really speak for themselves: the Passat running on diesel, for example, went nearly 150 miles further than its gasoline counterpart, and we found that to get the same number of miles from gas, you’d need to put an extra $20 per tank.

So, if you go through, say, a tank a week, you’re saving $1000 a year with a diesel-fueled vehicle.

It’s worth noting that not all diesels are equal; the Golf, for example, is more fuel-efficient with gasoline, and managed to get within a hundred miles of its diesel counterpart. Golf diesel drivers would only save $500 or so a year on the “Tank a Week” schedule.

It’s worth noting that if you tend towards mid or premium gas, diesel might become worth it: premium and diesel were essentially the same price.

Of course, you’d also have to factor in the extra money you’d have to pay for a diesel engine in the first place. This varies widely: Audi, for example, will only charge you about $3000 more for a diesel, while Volkswagen generally asks $6000 more. So the Audi A3 will cost a
bit more overall, but you’ll begin getting net savings in three years, while the Passat will take nearly six years. And the Golf? Well…that will take a decade.

In short, diesel might be able to save you money in the long run, but take a good hard look at the model of car you want and the premium you pay for a diesel engine. Compare how much you may save over time, and remember that fuel prices aren’t stable and can swing wildly.

If you’re looking for a heavy-duty vehicle, you should definitely consider a diesel. But if you just want to save money, you should probably consider driving a hybrid or smaller car with higher fuel efficiency instead.

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