Engineering Explained tackles the commonly asked question.

Ever since cars were equipped with components to monitor instantaneous fuel economy, driver and passengers have made it a pastime to get that number to read as high as possible. Yes, there are also those who revel in seeing how long that number can stay low, but even the most die-hard horsepower junkies love to watch that instant mpg figure climb, especially while rolling down a big hill.

To do so, you just throw the car into neutral and let it coast down the hill with the engine at idle, right? Actually, that’s probably not the best way to get the biggest return, whether you have an automatic or a manual transmission. It might seem like a no-brainer to most, but Engineering Explained took a Subaru Crosstrek out to some hills, and with our familiar narrator at the wheel, talked about why leaving the car in gear is actually going to be better in the long run.

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The very simple answer to this question is that it's better to keep the car in gear. When the transmission is still engaged – be it an automatic or a manual – the computer will detect acceleration without any throttle being applied, and it will cut fuel to the engine. Yes, the engine is still running but the drive line is turning it over as opposed to ignited air/fuel mixtures in the cylinders. If the car is taken out of gear, the computer will detect there’s no throttle input or gear engagement, and as such will supply enough fuel to the engine so it will idle.

This is actually visualized quite clearly in the video, with the Subaru maxing out its instant mpg meter at 99.9 while coasting in gear. With the car out of gear, the figure is still quite high, but it generally fluctuates in the 60 - 80 mpg range.

The video goes on to say that, for coasting over multiple hills, it’s more efficient to take the car out of gear so enough speed is built up to coast up the hills. While technically accurate, it’s not exactly practical, nor is it likely even legal given the speeds required to coast uphill for any significant difference. So for all intents and purposes, keep the car in gear at all times.

This is actually a subject that has led to more than a few arguments among car fans, so thanks Engineering Explained for clearing up this question on coasting.

Source: Engineering Explained via YouTube