Here is a table that perfroms the math to compute MPG (Miles Per Gallon):
this means in cell D3 type (or paste from this post) "=(C3-C2)/B3" without the double quotes
to "expand" the formula, more properly known as fill then formula, select the cell D3, grab (click and hold) the little circle at the bottom-right of the cell and drag down as shown. One comment I have is that my car has a an odometer and two trip counters (A and B). I use "B" to keep track of mileage since the last fillup so rather than typing Odometer readings in I type the trip mileage.
In this case you would change the name of column C and the formula would be: