For my entire ecodriving life, I've been coasting in neutral for as far as I could before having to stop. But recently some people have been saying that it's more efficient to coast in gear and just downshift as I slow, saying that "When the car is coasting out of gear, the ECU provides fuel to your engine to maintain an idle.
When your car is in gear, and your foot is off the gas, and you are coasting/slowing to a stop, the ECU signals a zero pulse width (turn off the fuel) signal to the fuel injectors (meaning it turns off the fuel supply). Therefore, you use ZERO gas when coasting in gear." and cited this article.
The ECU mode in question is called DFCO (Deceleration Fuel Cut Off), and the condition where your throttle is closed but you're rolling in gear above idle is engine braking (even if it's not the large amount of engine braking you think of for that term, such as you might use to control speed descending a mountain).
Unfortunately it's not as cut-and-dry as we would like it to be. The behavior differs significantly from one vehicle to the next. There are dozens of conditions that the ECU uses to decide whether or not to DFCO. Accessory usage, temperature, recent shifts, etc. OBDII devices like the ScanGauge II can give you a basic idea of your DFCO behavior but sometimes OBDII data doesn't refresh fast enough to really understand. I used a meter connected to a fuel injector wire to see when the injectors were truly off with good accuracy. If you don't get DFCO while engine braking then instead you'll spend more fuel as fuel is injected to keep up with the higher RPM.
It's also not a substitute for neutral coasting. DFCO and neutral coasting are different tools for different jobs...
- When you intend to reduce or control speed, DFCO is better than coasting and braking.
- When you are doing Pulse & Glide or otherwise not intending to reduce or control speed, coasting is the more efficient option. Just think about the energy; with DFCO you turn the engine more revolutions. Every revolution costs a small amount of energy. That energy comes out of your inertia. It is more efficient to burn fuel at idle RPM than to eat inertia at higher RPM. This is not just theory; it is consistent with data collected in many gaslogs.
Of course, coasting isn't perfect. There can be legal or even safety issues with coasting that you don't have with DFCO. It is commonly illegal to coast downhill in neutral or with the clutch disengaged.