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ex machina

Many drivers wonder why their car's fuel economy is worse than the official rating on the window sticker. The answer takes us into interesting areas of engineering, corporate strategy, and human behavior.

How you drive is the single biggest factor when it comes to fuel economy - cruising at 140 km/h and drag racing from light to light can easily double or triple consumption. But there are also issues with the tests that are used to produce a car's fuel consumption numbers. Although they must be approved by government regulators, the tests are performed by manufacturers, who have a vested interest in publishing low consumption figures.

As you might expect, the tests are designed to make a car look good. For example: they use moderate speed and acceleration profiles that maximize efficiency, but are out of step with the habits of most drivers. This discrepancy can be particularly significant with turbocharged cars, which often deliver far worse fuel economy than their official rating.

So how do you maximize your fuel economy? Here are some proven methods:

  • Drive at the speed limit. At highway speed, the majority of the fuel you burn is used to overcome aerodynamic drag. Because drag increases as the square of velocity, the fuel economy hit is more dramatic at higher speeds. The average car will achieve its advertised fuel economy at 90 km/h. At 105 km/h, fuel economy will be about 8 per cent worse. At 112 km/h, the impact on fuel economy is 17 per cent. Going up to 130 km/h will increase fuel consumption by about 30 per cent.
  • Accelerate gently and keep your speed constant. Your engine is most efficient at moderate throttle openings. Rapid acceleration and continual speed changes burn extra fuel.
  • Use cruise control. In almost every case, the cruise control system will do a better job of optimizing fuel economy than a human driver. Maintaining a steady speed minimizes the changes in throttle opening, which minimizes fuel burn. Although the cruise control system will vary the power setting to maintain speed, it’s programmed to make the changes gradually, which conserves fuel.
  • Take off the roof rack. Because it generates aerodynamic drag, a roof rack can sharply increase fuel consumption. The impact of a rack depends on its configuration, and your driving speed. In a documented test, Consumer Reports found that a 2013 Honda Accord with no rack got 42 mpg U.S. at 105 km/h. An empty roof rack cut the Accord’s fuel economy to 37 mpg. When two bikes and a wind deflector were added to the rack, the Accord’s fuel economy fell to 27 mpg.
  • Use air conditioning on the highway. At highway speed, most of your car’s power is used to overcome aerodynamic drag, so rolling down the windows is a bad idea, because it disrupts the airflow over the car. Many drivers believe that air conditioning hurts fuel economy, but they’re only partly right – the load imposed by the air conditioning system does burn extra fuel, but at highway speed, the drag created by open windows burns even more.
  • Go au naturel around town: at low speeds, where aerodynamic drag is minimal, turning off the air conditioning system and opening the windows will save fuel.
  • Avoid ethanol. Made from plant materials like corn and grass, ethanol is added to fuel as part of ongoing efforts to reduce oil dependence and greenhouse-gas emissions. Because it contains less energy than gasoline, ethanol has a direct impact on fuel economy. At the low concentrations used in fuel blends like E10 and E15, you will take a fuel economy hit of three to five per cent. At high concentrations such as the E85 blend used in Flex-Fuel vehicles (FFV), fuel economy can be as much as 27 per cent worse than with 100 per cent gasoline.
  • Avoid idling. You get zero mpg when you’re not moving. If you think you’ll be stopped for more than a minute, turn off the engine. (Restarting uses about ten seconds worth of fuel.) If your car has an automatic start-stop feature, this will take care of itself. If you don’t have automatic start-stop, limit your engine shut downs to about a dozen per day to avoid excess wear on the starter system.
  • Check your tire pressure. Under-inflated tires increase rolling resistance, adding to fuel burn.
  • Eliminate unnecessary weight. Every kilogram of luggage or equipment you put in your car increases the amount of power (and fuel) it takes to accelerate and maintain speed. Taking out things you don’t need saves fuel.
  • Choose the right car. The most energy-efficient cars available today are electric. But their capital cost is still too high for most buyers. Hybrids are a smart choice if you keep your cars long enough to offset the added initial cost (my wife and I just bought a new Toyota Prius hybrid, partly because its extreme efficiency will more than repay the price premium over several years of ownership.) No matter what type of powertrain you choose, efficiency will be optimized if you buy the smallest car that will suit your day-to-day needs. If you need a pickup truck or full-sized SUV to haul equipment or children, go ahead – but if you can make do with something smaller, the efficiency payoff will be ongoing.

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