Coasting in Neutral Wastes More Fuel Than Idling in Gear

A 1976 Buick LeSabre with a 350 cubic-inch V8, tested on a level road at a steady 30 mph, consumed fuel at three different rates depending on what the driver did. Steady cruising burned 0.030 gallons per minute. Coasting in drive burned 0.012 gallons per minute. Coasting in neutral burned 0.017 gallons per minute, according to a fuel economy training document from the Iowa Department of Transportation. In that carbureted car, coasting in gear used less fuel than coasting in neutral. But the difference was small, and the neutral figure was still lower than steady cruising.

Now put a modern fuel-injected car on the same road. The gap gets wider, not narrower. When you lift your foot off the accelerator and leave the transmission in gear, the engine control module detects the closed throttle and the elevated intake manifold vacuum, and it shuts off the fuel injectors entirely. The driveline keeps the engine spinning. Fuel consumption drops to zero. When you shift into neutral, the engine disconnects from the driveline and has to burn fuel to maintain idle. The amount is small, but it is not zero.

The claim holds for modern fuel-injected cars: coasting in neutral uses more fuel than coasting in gear. The mechanism is deceleration fuel cut-off, a feature that shuts off the injectors when the throttle is closed and the engine is being turned by the wheels. But the old advice to coast in neutral was correct for carbureted engines, which could not cut off fuel delivery during deceleration. The rule flipped when the hardware changed.

How the carburetor handled coasting

A carburetor delivers fuel by using the engine’s intake vacuum to draw gasoline through a jet. The throttle plate controls how much air flows, and the fuel follows. When you close the throttle to coast, the engine is still turning, still pulling vacuum, and still drawing fuel through the idle circuit.

The fuel flow at idle is calibrated to keep the engine running smoothly, not to minimize consumption. There was no mechanism in a standard carburetor to detect coasting and stop the fuel flow. A 1945 patent from Carter Carburetor Corporation described a fuel cut-off device that used a spring-loaded valve to block fuel when intake vacuum exceeded a certain threshold, but this was an add-on, not standard equipment. Most carbureted cars kept feeding fuel at idle rpm during coasting.

That is why the old advice made sense. If you coast in gear with a carburetor, the engine is still turning fast, and depending on the carburetor design, it may draw more fuel than it would at idle. If you shift into neutral, the engine drops to idle speed, and the fuel flow drops with it. The 1976 Buick data shows the effect: 0.012 gallons per minute in gear versus 0.017 in neutral. Wait, that shows the opposite. In that particular test, coasting in gear used less fuel than neutral even with a carburetor, likely because the engine was still turning fast enough that the idle circuit wasn’t the dominant fuel path. The document itself notes that coasting in neutral was a “common fuel saving strategy” back in the carburetor era.

What changed with electronic fuel injection

Electronic fuel injection replaced the carburetor with a system where the engine control module (ECM) decides how much fuel to inject and when. The ECM monitors throttle position, engine speed, intake manifold vacuum, and other sensors. When it sees the throttle closed and the engine speed above a threshold, it can shut off the injectors entirely.

This feature is called deceleration fuel cut-off, or DFCO. It has been standard in production vehicles since at least the early 1980s. A 2018 SAE technical paper on coasting in neutral states that DFCO “exists in all current vehicle powertrain controllers.” The paper’s authors note that the duration of a neutral coast “must be long enough to attain more fuel economy benefit than Deceleration Fuel Cut-Off (DFCO) can bring.”

When DFCO is active, the engine is still turning, still pumping air, and still running the accessories, but the fuel injectors are off. The vehicle’s momentum drives the engine through the transmission. Fuel consumption is zero. The dashboard’s instant fuel economy gauge maxes out, often at 99.9 mpg or higher.

When you shift into neutral, the transmission disconnects from the engine. The engine drops to idle speed, and the ECM resumes injecting fuel to keep it running. The idle fuel flow is low, but it is not zero. A forum discussion on the Grassroots Motorsports site describes the situation plainly: “most injected cars shut down the injectors when coasting in gear.”

What the measured figures actually show

Engineering Explained tested a Subaru Crosstrek on a downhill grade. With the car in gear and coasting, the instant fuel economy meter maxed out at 99.9 mpg. With the car in neutral, the meter fluctuated between 60 and 80 mpg, according to the test reported by Motor1. The in-gear figure represents fuel cut-off. The neutral figure represents idle fuel consumption.

The gap is not huge in absolute terms. Idling fuel consumption for a typical four-cylinder engine is roughly 0.2 to 0.3 gallons per hour, which works out to about 0.003 to 0.005 gallons per minute. Over a 30-second coast, that is 0.0015 to 0.0025 gallons. At $3.50 per gallon, the cost is less than one cent. Over a long downhill grade lasting several minutes, the difference could add up to a few cents.

But the comparison is not just about the absolute fuel saved. It is about which method uses less. In gear, the fuel used is zero. In neutral, it is slightly more than zero. Zero is less than any positive number.

Why the old advice persists

The advice to coast in neutral made sense for decades. Drivers who learned to drive before electronic fuel injection was universal learned a technique that worked on the cars they drove. The technique was passed down, and it became part of the folk knowledge of fuel-saving driving.

A 2025 article on Jalopnik notes that “anybody who’s been around cars for a while has probably picked up some questionable advice along the way.” The article explains that the logic behind coasting in neutral “isn’t totally wrong-headed, but it also doesn’t take into account the level of technology that’s present in modern cars and trucks.”

The advice is not just outdated. In many places, coasting in neutral is illegal. Virginia law states that “the driver of any motor vehicle traveling on a downgrade on any highway shall not coast with the gears of the vehicle in neutral.” Similar laws exist in other states, though enforcement is rare. The safety concern is that without the engine connected to the driveline, you lose engine braking, and you have to rely entirely on the friction brakes to control your speed on a long descent. Brake fade becomes a real risk.

There is one narrow condition where neutral coasting can save fuel in a modern car. A 2018 SAE paper on using ADAS to improve coasting in neutral found that if you need to coast for a long distance, such as approaching a stop from far away, and you want to preserve momentum to roll up the next hill, neutral coasting can reduce the speed loss caused by engine braking. But the same paper notes that the coast must be long enough to overcome the DFCO benefit. For most driving situations, the in-gear coast wins.

The practical takeaway is simple: leave the car in gear when you coast. The engine will cut fuel on its own, and you will use less than you would idling in neutral. The rule that was true for your grandfather’s carbureted car is not true for the fuel-injected car you drive today.