Many people believe that changing into neutral down hills will save fuel, but in most cars and in most scenarios this is in fact not true.
Now the folk at Engineering Explained have taken it upon themselves to bust this myth once and for all in the video below, which was recently posted on YouTube.
The theory is simple. In modern fuel injected cars, the fuel supply is actually shut off completely when the car is engine braking in gear, but if you slot it into neutral it will keep pumping fuel as it assumes the car is stopped and idling. This applies to both manual and automatic gearboxes.
It’s a different story with carbureted engines, however, which will continue to pump fuel through the engine while coasting in gear, but even then the impact on overall efficiency will probably to be negligible.
But it's not a completely one-sided argument. The video presenter also illustrates that one might save more fuel in neutral when there are a series of smaller hills ahead and the faster cruising speeds attained in neutral would get you up the other side without using engine power.
Yet ultimately you would really need a heck of a lot of downhill driving to make a noticeable difference, and ultimately common sense dictates that you’re also a lot safer staying in gear, as not only is the car a lot more controllable, but you might need that engine power in an emergency.
So the answer is simple... just stay in gear!