The ONLY way of talking reliable figures would be to use figures from a brim-to-brim fill up. Otherwise relying on the OBC you will find that you will also ahcieve 99.9MPG in certain situations - if you beleive that, then well I guess the moon is also made out of cheese and the Earth is indeed flat.
Talking OBC is pointless IMHO.
ok done it your way and got 36.8 MPG that was driving carefully but not totally in a hybrid style for the whole time, this was over 253 miles, 31.17 litres, unless my calculation are wrong?