Werner's Blog — Opinion, Analysis, Commentary
Fuel Efficiency and Your Driving Style

As gasoline prices in Vancouver have hovered near \$1.65 per liter over the last few weeks, motorists and politicians have been wondering what to do about it. The reality is that there are a multitude of factors that determine fuel prices, and politically there is very little that can be done about it. Calls to reduce fuel taxes are populist nonsense—it is an ineffective tool because at best it could shave off a few cents; it would jeopardize much-needed revenue for road maintenance and public transit; and it would ultimately benefit oil companies more than motorists if demand is relatively price-inelastic. The best that motorists can do are two things. In the long-term, drive a more fuel-efficient vehicle. In the short term, improve your driving style.

‘Smart driving saves more gasoline than most motorists think.’

Driving style matters greatly, a fact that many motorists do not really appreciated. Judging what I see every day in Vancouver traffic, the need to get ahead a few meters while exceeding the speed limit is an urge that many cannot resist. Vehicles that pass me never seem to gain much: I often catch up with them at the next traffic light. Aggressive driving, often excused as "sporty" driving, seems to confuse city roads with a race track. Average traffic speed is determined by traffic conditions, not an individual's driving style. Aggressive driving is more likely to get a driver into an accident, but it will also hurt that driver's wallet because of higher fuel consumption. So what can smart drivers do to reduce fuel consumption? Natural Resources Canada has put together a page with excellent tips on how to improve your driving behaviour and reduce fuel consumption:

1. Accelerate gently: rapid acceleration tends to require more fuel because the engine operates inefficiently. However, don't accelerate too slowly either; there is an optimal rate of acceleration that keeps fuel efficiency low.(*)
2. Maintain steady speed: use cruise control where possible, but also avoid erratic speed changes, lange changes, and unnecessary passing maneuvers in traffic.
3. Anticipate traffic and coast to decelerate: look ahead and keep a comfortable distance to the vehicle ahead of you; notice well in advance when it's time to coast to a slow down instead of breaking.
4. Avoid high speeds: the most fuel-efficient speed is usually around 70-80 km/h, and traveling at 120 km/h instead of 100 km/h tends to use about 20% more fuel.

There are additional measures that help, such as removing unnecessary weight and airflow impediments (e.g., unused roof racks), keeping your tire pressure at the optimal level, and paying attention to the fuel-consumption indicator if you have one. Additionally, some newer vehicles offer a start-stop automatic that will stop the engine instead of idling it. (I had reported about it in my August 2016 blog.) The techniques mentioned above can cut fuel consumption by as much as 25%, claims Natural Resources Canada.

Are the 25% in fuel savings advertised by Natural Resources Canada overly optimistic? What is the empirical evidence? Researchers at the Oak Ridge National Laboratory conducted a study (Thomas et al. 2017, see below) that shows convincingly that driving style matters, and they conducted the tests both on gasoline-powered and hybrid-electric vehicles. They found fuel savings of 10-40% in stop-and-go traffic and 15-30% at highway speeds. Interestingly, HEVs are somewhat more sensitive to driving style than conventional cars because the performance of HEVs depends on how well regenerative breaking works—and that depends on driving style. The empirical evidence suggests that fuel savings are real: follow the guidelines above, and you can reduce your fuel costs.

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Lastly, a note on the physics of acceleration for the asterisk above. The energy $$E$$ required to accelerate from zero to speed $$v$$ is equal to $$E=(1/2)\cdot m\cdot v^2$$. So the energy you require is proportional to mass. The heavier your vehicle, the more fuel you need to accelerate. It appears as if the energy required does not depend on the rate of acceleration, just the final speed, so that it may not matter how much you accelerate to gain the final speed. This is unfortunately not so simple because engine performance is related to acceleration, gear ratios, and optimal RPMs near peak torque. So what is the optimal acceleration depends a little bit on your car, but for most vehicles with automatic transmission, avoid high RPMs as you want your car to shift to the higher gears sooner.