Calculating MPG (1 Viewer)

I know this must have been covered before but I'm really struggling with this.
I want to calculate MPG between fill ups for one vehicle only.
I have a table that has the following fields:

fuel ID Gallons Odometer TankMiles
1 12 19340
2 6 19680
3 8 20458

How do I calculate the TankMiles column.
I know I have to get the Odometer value from the previous record then subtract that from the latest one.
I am trying to use the expression builder to create a new field called TankMiles but i am struggling with formula and syntax.
I am sorry to burden you all with such a newbie question but I have looked all over the internet and I think I am struggling as much with the question and just can't find the answer.

Thanks

Use a subquery. See http://allenbrowne.com/subquery-01.html#AnotherRecord