Speedo up means eco down

We’re not talking swim suits here, we’re talking about that gizmo that tells your how fast you’re going. Most folks know that fuel economy drops in very slow, stop-and-go driving as well as in very high speed driving. Surveys have confirmed these effects for decades, but as automotive technology advances and overall gas mileage improves, are the impacts really so bad?

Airflow over a model year 2013 Ram 1500 pickup truck

Better aerodynamics boosts highway mileage and all vehicles have gotten a lot more streamlined in recent years. The Dodge Ram, for example, has progressively moved from traditional, boxy pickup styling to a much more sculpted look. The 2013 Ram 1500 boasts a best-in-class drag coefficient of 0.36, which is engineering speak for slipping a lot more easily through the air than many sedans of yesteryear.

Nevertheless, the laws of physics dictate that as speed goes up, drags goes up by a lot more. Recent tests at Oak Ridge National Laboratory (ORNL) confirm that a large decline in fuel economy still occurs as the speedometer moves from 50 to 60 to 70 miles per hour (mph) and beyond. As Green Car Congress reports, ORNL researchers tested nearly two dozen vehicles in their laboratory at carefully monitored speeds, and they also analyzed a proprietary data set covering over four dozen more vehicles supplied by research colleagues at Chrysler.

The results confirm a marked and ever-increasing MPG drop-off as velocity rises. Going from 50 to 60 mph cuts efficiency by an average of 12 percent and another 10 clicks up the speedo dings fuel economy by 14 percent more. Stepping on it from 70 to 80 mph drags miles per gallon down by another 15 percent.

Although the report doesn’t name names, some vehicles fared better than others. For a few cars, fuel economy wasn’t that sensitive to speed, but others fared worse. Some revealed a very marked decline in the highest speed range tested, from 70 to 80 mph.

The most sensitive vehicles were those whose engines are programmed to run rich at high power, which keeps the catalytic converter from overheating. That causes fuel burn to jump and also causes a huge, disproportionate spew of tailpipe pollution. Although a rich mixture can protect the catalyst, it also destroys its pollution-scrubbing effectiveness.

The bottom line? Don’t take today’s more fuel efficient designs for granted. How you drive still matters a lot for eco-friendliness, and pushing too hard on the pedal definitely degrades the environmental performance of your car or truck.


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