Many people who own
ScanGauges wonder why the "fuel cut" on deceleration doesn't show up in the instantaneous numbers...
I did ask about fuel-cut specifically, and the answer was: cars which
do have overrun fuel cut do not all report it the same way. IE - some continue to calculate and report injector pulse width despite the injector(s) itself being inactive (he speculated that this is so "reignition" will be smooth).
On that point then, you should believe what your service manual says about fuel cut-off on overrun, not the
SG. (Which raises a question: will an auto tranny will do fuel cutoff? What would keep the motor spinning if the TC isn't locked up?)
He also emphasized that the difference between the
SG and the other OBD products on the market is the amount of time & programming effort his company has put into the fuel consumption calculations, so he's understandably protective about revealing specifics.
He also said that a significant amount of time is spent "debugging" cars which don't all follow the same rules, in terms of OBD2 communication. IE the cars that follow the "expected" programming do show "9999" mpg on fuel cutoff. (Mine happens to be one of the cars that doesn't, even though my service manual says it has that feature). He also estimates the effect on accuracy of
not factoring that condition into fuel economy calcs is probably about 0.1%.