Zombie thread alert...
The
2007 Civic is doing the same thing: no 9999 mpg instant reading when the engine is killed.
I thought it might be related to keying off, so I also verified it two other ways:
1) with a kill switch on the fuel pump circuit, and,
2) by stalling the car with the clutch at the top of a hill and then letting it gather speed down the hill (instant MPG goes up as the car picks up speed).
This is with a SG2.
Have to dig a bit more...