Hi Billymick,
You can get as complicated as you want doing this but in the simplest terms
it can be done with a couple of columns of data:-
start miles end miles Distance Fuel added mpg
1000 3000 2000 100 20
Distance is a formula = B2-A2
mpg is also a formula =C2/D2
Start miles is taken from you odometer
Fuel added is what you put in at the pump
So the next time you fuel up, start miles would be 3000 and end miles would
be you odometer reading and you would drag the formula down.
Mike