9999 mpg is seen when you let go of the throttle while in gear , and the engine goes into DFCO - decelleration fuel cut off.
No fuel is being used as the flow is cut off, but you're still covering distance.
In Maths you should get a division by 0 error
Set the cut off, and start calibrating the SG when you fill up (menu item FILLUP).
Compare the SG's estimate of the fuel used, with the actual volume you tanked.
Correct the SC using the < > buttons, but only correct it halfway towards the actual value - otherwise it may swing the other way on the next fill up.
The correction will quickly get smaller over a few tanks, and the readouts will get rather accurate.