Consider the following three fill-ups:

Date	Odometer	Amount	Full?
Jan 1	1500 mi.	25 gal.	Yes
Jan 15	1950 mi.	20 gal.	Yes
Jan 25	2505 mi.	28 gal.	Yes

Between Jan 1 and Jan 15, you travelled 450 miles (1950 - 1500) and consumed 20 gallons of fuel (the amount of fuel it took to re-fill the tank). Therefore, your fuel economy for that time period is 450/20 = 22.5 MPG.

Similarly, between Jan 15 and Jan 25 you travelled 555 miles (2505 - 1950) and consumed 28 gallons of fuel. Your fuel economy for that time period is 555/28 = 19.7 MPG.

This calculation requires that you fill your tank. When you don't fill your tank the software will not know exactly how much fuel you've consumed since the last fill.