Having some fun with my SG2 at the moment.
On every tank it is spot on with the guess at how much fuel is needed - it has been "adjusted" so it things gallons are imperial ones and not the colonial mini ones
But on individual trips it can be odd. I filled up at the end of last week and did a 4 city round trip in Scotland. Most of it was motorways and and the AVG for the trip was ~75MPG (yay!). But then when I filled up the measured MPG came down to ~65 (which is normal) and the SG2 guessed the fuel needed spot on again.
So how did it get an AVG so high when all other figures were correct ? I've checked the MPH measurement against the speedo and it is also correct.