Living Columns & Blogs

Black gold and mileage

Crude oil is a mix of stinky chemicals. But smell as they may, the liquids in petroleum are vital to us.

An alert reader of these columns recently queried me about what we get out of a barrel of oil -- and what those products mean in terms of miles actually driven on American highways.

Petroleum engineers “take apart” crude oil in refineries, separating it into different chemicals. Roughly speaking, a barrel of oil -- which is a bit more than 40 gallons -- gives us 20 gallons of gasoline, 10 gallons of diesel, and five gallons jet fuel. (The stuff in jet fuel is also known to us old biddies as kerosene, but “jet fuel” certainly sounds more modern.) The remainder of the barrel is material such as heavy oils and liquefied petroleum gases, the price of which you’ll see in business reports as “LPG.”

But what does a barrel of oil do on our roads? I’ll answer that using the example of a recent model of the Jeep Grand Cherokee. I’m choosing that vehicle simply because it can be purchased with a gas engine, a diesel engine, or a “flex-fuel” engine that runs on either gasoline or 85 percent ethanol. I’ll look only at city mileage -- the lower end of the range you’d likely get with the vehicle. All my numbers are from a government Web site, based on the 4-wheel drive, 2008 model of the Grand Cherokee with an automatic transmission. I made certain choices about engine size in what follows. You can check variations on the figures or look up your own vehicle at www.fueleconomy.gov.

The Grand Cherokee gets 17 mpg in the city with a diesel engine and 13 mpg with a traditional gasoline engine. In other words, diesel gets you farther down the road than gasoline does. For an individual person, that’s all that may matter.

The downside of diesel includes engine “clatter” and challenging start-ups at bitter temperatures. The upside -- beside better mileage -- is considerably more basic power. That’s important if you tow a big motorboat up and out of deep river canyons, as this Rock Doc has on summer evenings. (Can you say, “boiling over”?)

If you want to think about what a single barrel of crude does on our city streets, the Grand Cherokee figures work out like this: A barrel of crude powers a diesel Grand Cherokee about 170 miles (because 17 mpg times 10 gallons of diesel is 170 miles) and that same barrel also powers a gasoline Grand Cherokee about 260 miles (because 13 mpg times 20 gallons of gasoline is 260 miles). That’s a total of 330 miles in the two vehicles from a single barrel of petroleum.

Moving heavy vehicles over city streets more than 300 miles is no mean feat, which is why we have a heavy national dependence on crude oil.

Perhaps you want to lessen our American addiction to foreign oil. If you buy a flex-fuel engine in your Grand Cherokee and run it on “E85,” the fuel made of 85 percent ethanol and 15 percent gasoline, you get only 9 mpg in the city. That single digit mileage reflects the fact that ethanol doesn’t contain nearly as much energy as gasoline or diesel.

We Americans make ethanol from corn, and the process of growing, harvesting and processing the corn uses a lot of energy from coal, from natural gas and, indeed, from petroleum. So ethanol is hardly free of fossil fuels, including foreign oil.

If you want to check out vehicles much more fuel efficient than Grand Cherokees, the same basic link at can take you to many choices. With fuel prices inching up, this geologist thinks that type of research will be increasingly wise.

The day is likely coming when many of us will commute to work in cars powered primarily by electrical energy, not liquid fuels. Plug-in hybrids will likely reach the average consumer in a year or two. Fully electric cars can be purchased now, but widespread use depends on next-generation batteries. Stay tuned for news.

Meanwhile, the stinky chemicals in a barrel of crude oil will remain central to our daily lives.

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