Wiser Driving

I’ve heard that the way you drive an electric vehicle (EV) can affect range dramatically. Does the same apply to fuel economy for engine-driven vehicles? Can you give me some basic pointers on how to drive so I use less energy and create less pollution?

James Fallow • Big Pine, California

Many factors affect driving range, but air drag and weight are certainly two of the most important. For an EV moving at less than 30 mph, it’s the weight of the vehicle that kills driving range; as speeds increase beyond 35 mph, air drag takes over as the biggest culprit of dragging down fuel economy.

Some idea of air drag’s insidious nature can be gained from data for the RAV4 EV—one of the most-studied EVs ever built. At 45 mph, the car can travel almost 150 miles on a single charge; at 60 mph, driving range plummets to about 100 miles (just imagine what happens at 80 mph).

In the case of a conventional internal-combustion-engine (ICE) vehicle, gains in fuel economy are there for the taking—if you’re willing to drive at a more leisurely speed. My 1993 Dodge minivan delivers its highest fuel economy—29 mpg—at a constant speed of 45 mph. (For safety reasons, I suggest not driving at this speed on the open highway.) When I dare to keep up with traffic on the Michigan interstate (80 mph), my minivan’s fuel economy drops to about 17 mpg.

Stop-and-go city driving also reduces fuel economy for ICE-based vehicles. This is a consequence of the operating characteristics of typical engines that are designed to operate at higher loads (and, hence, higher driving speeds), and the need for constant acceleration and deceleration. Most hybrid-electric vehicles have circumvented these problems and actually do as well, if not better, in the city as on the highway.

You can improve your city mileage with an ICE-based vehicle if you drive more intelligently. Learn how to coast, rather than braking, into a stop, and time traffic lights so you keep moving at a relatively constant speed. These measures will help increase your city fuel economy (as well as increase the time between brake replacements). Likewise, mountain driving offers a number of challenges to fuel economy. Here again, coasting (when possible) and driving slower (when no one is tailing you) will save fuel and reduce pollution.

Another means of saving fuel is to consider carpooling. If you put four people in one car, you’ll cut pollution and fuel consumption by about 75 percent compared to four people driving their individual cars. Now that’s impressive!

Dominic Crea • Institute for Sustainable Energy & Education