[ennored]

Original thread is old, but since it has been brought up, a couple points.

Scangauge infers DFCO, there's a setting for the throttle angle it uses to show 9999 MPG. Looks like it assumes it's in DFCO if it's open loop and at a low throttle angle. From the manual:

Set the Fuel Cutoff Level
Some vehicles will turn the fuel injectors off while
coasting – this is known as Fuel Cutoff. ScanGauge
attempts to detect the fuel cutoff condition by
comparing the open/closed loop indicator and the
throttle position.

Remember the Scangauge does not know fuel flow. It takes airflow and multiplies it by the stoichiometric air fuel ratio. (Don't know why they do it this way, fuel flow data is in the data.)

In my experience (big 3, last 25 years) DFCO is used for fuel economy. We had to be careful with it early on as it could make emissions worse. We carefully control to stoich to keep the cats working at high efficiencies, then DFCO kicks in and pumps them full of oxygen. We've gotten better about returning from DFCO, but it's still primarily a fuel economy tool.

And, as to whether a certain car, on a certain day, during a certain drive cycle, is doing DFCO. I have a phrase I use: "calibratable beyond explanation". That is, there's a zillion inputs that can and are used to determine if the fuel should shut off. It's not just pedal position. There's throttle position, airflow, engine temp, battery state of charge, gear position, torque converter state, trans temp, engine speed, road speed, ambient temp, ambient pressure, purge state, AC state, phaser position, codes present, tests running, phase of the moon, and more. Blipping the throttle and watching for the fuel to shut off isn't going to tell you much of anything.