fuel computers normally can be set-up to give fuel usage for a trip you are going to make, so you can see how much you are using, but mpg will depend on your driving style, and if you are stop/starting around town or on motorway at a constant speed and what load your car is carrying. wear & tear will cause some loss of mpg, but if you give it a bit of tlc, i.e regular service, tyre pressures correct, pull away steady (not like jenson button) this all helps to get a bit more mpg.
hope it helps
August 2010