Does this advice apply to all cars?
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Accelerating slowly in cars with automatic transmission or CVTs allows them to shift to a higher gear (or lower gear reduction) for more of the time and distance they are accelerating compared to accelerating fast. Higher gears are better for fuel efficiency because the engine spins slower to make the car go the same speed than in lower gears. Just like how riding your bike in higher gears uses less energy per mile than lower gears.
The most efficient way to slow down in modern cars is to not touch the brake or accelerator pedal at all and let the car slow down on its own. This is called engine braking and modern cars cut of the flow of fuel to the engine entirely while you’re doing it. Manual transmission cars can coast in neutral as well, but the engine will still consume fuel just to maintain idling RPM. A lot of driving situations keep you from slowing down like this. However, coasting in neutral, engine braking or gentle braking still improve fuel economy because, if you’re decelerating very slowly like this, you probably didn’t use a lot of fuel getting up to an excessive speed in the first place. Gentle braking or coasting necessarily means you’re lowering your fuel consumption to get up to speed before you ever start slowing down. And the next traffic light might change or the cars ahead of you might start moving again if you’re going slower compared to when you’re going faster. So you need less fuel to get back up to speed in these situations.
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