The way a driver operates a car can produce very different fuel-economy numbers, and when driving downhill, two notable techniques come into play.
The first is leaving the car in gear and using so-called engine braking to provide at least some deceleration. But many believe putting a car in neutral uses less fuel, followed by use of the brakes to slow as needed.
Is one way more efficient than the other? There's an explanation, but it depends on the particular driving situation.
Most of the time in normal driving situations, just leaving the car in gear is the best way to maximize fuel economy.
Per a video from Engineering Explained, the engine cuts the fuel injection when the car moves down a hill; at various times, the engine will use no fuel at all to power the engine, since the wheels themselves—connected to a transmission—make the engine spin.
Contrast this with putting the car in neutral and leaving the engine running. Video viewers will notice fuel economy actually drops while moving downhill.
Since the engine is effectively at idle, fuel enters the combustion chamber to keep the engine powered, unlike the engine-brake scenario.
The amount of fuel used is minimal, to keep the engine at idle rpm, but it does result in significantly lower fuel-economy figures while in neutral.
However, there is a time when putting a car in neutral and coasting will return greater fuel efficiency—although it requires the driver to plan ahead.
If a particular driving route features several hills of various grades, neutral will let the car build more momentum to tackle, say, a big hill, then a slightly smaller hill, followed by one last hill.
In an engine-braking scenario, the car would carry less speed up to the next hill, so the driver would need to accelerate, using more fuel.
Coasting in neutral to gain speed should only be done safely and within applicable speed limits, of course.
2016 Jaguar F-Type S V-6 manual
Another and rather more common scenario is approaching a traffic signal.
If a signal is about to change from red to green, coasting rather than engine braking will keep the vehicle at a higher speed and so use less fuel than slowing the car more and then having to accelerate back up to speed.
This analysis does not take into account safety, defensive driving, or other factors; it only looks at fuel efficiency.
But understanding how a particular vehicle works, with a little more thought behind the wheel, it's possible to boost fuel economy—at least slightly—with these simple tips.