Getting Best MPG: Coast in Neutral or Leave in Gear?

When it comes to maximum fuel efficiency, is it better to coast in gear, or put it in neutral?

Engineering Explained is a great YouTube channel full of videos answering all manner of automotive and engineering questions, and explaining them in a manner anyone can understand. Last month, host Jason Fenske set out to answer the sort of question car guys have argued about forever, and thanks to modern technology it only took him five minutes. So what saves the most gas, coasting in gear, or engine braking?

In most modern cars, the computer controlled fuel injection will shut off the fuel entirely in certain situations. In order to run the accessories, air conditioning, and charging system at idle, the engine needs a small amount of fuel. If you put the car into neutral, or depress the clutch, the engine goes down to idle speed. But, if you leave the car in gear and let momentum turn the motor through the wheels, driveshaft, transmission, etc, the computer shuts off all the fuel and your MPG goes up to 99.9 (or whatever the max is in your car).

When rolling up to a stop sign, or rolling down a big hill, leaving the car in gear is your most efficient gambit.

However, if you are driving in hilly terrain, you can gain a little bit of extra momentum and energy by letting the car pick up speed on the way down so you can get up the next one. Disengaging the engine, with the clutch or gear shift, will allow the car to get up to maximum speed. You really won’t notice much difference unless you have a particularly heavy vehicle, like a G-Wagen, which needs a lot of power to push the weight and barn door aerodynamics up a hill.

Of course, this is all still hypothetical stuff based on a perfect world, where you don’t have to obey speed limits, or power through a corner. When driving a manual transmission car from now on, we’re going to leave it in gear, and not push in the clutch and coast whenever possible.