[Occasionally6]

If you are slowing down anyway, so braking up to a red light or maintaining speed down hill for examples, DFCO (for Atheria, that's Deceleration Fuel Cut Off) must be better, simply because there is no fuel being consumed.

The alternative is to consume fuel idling the engine (vs one that is being motored), and because of the opportunity cost of not converting kinetic energy into electrical energy, hydraulic pressure (power steering) or cooling work (A/C).

In the case of the stoplight, you can either travel quickly up to the stop point and wait there with the engine idling, consuming fuel, or you can brake a little harder, early in the slowing down process, get into DCFCO, and "wait" with the fuel shut off and the engine accessories being driven, recovering kinetic energy, while still moving. If you do it right you will also maintain some momentum through the light when it turns green. Maintaining speed down hill simply by braking is, of course, a total energy loss.

Reading between the lines, I'm guessing that you guys are prepared to vary your speed a lot. (I'm not.) OK, if you are prepared to drop your speed way down going up hill you might gain more by avoiding the less efficient energy conversions and simply converting kinetic energy to potential energy with little loss.

How much drag the engine has with the fuel shut off will depend on the engine, what accessory load the engine is driving and on what the engine management does. Particularly with drive by wire throttle (DBW) control, but also with the way the idle control bypass air is managed with a cable throttle, the pumping loss of the engine will be minimised with DCFCO active by allowing more air into the engine than would be the case with the engine switched of and in gear. Any Exhaust Gas Recirculation (EGR) will also have an effect. You might not be getting a valid comparison if you're only cutting the injectors with a kill switch. What happens when you also open the throttle?