If you are in the US, it is usually miles per gallon, so that would be:

fuel economy (Mpg) = Distance Driven (miles) / Fuel Used (gallons).

In countries that use the metric system, they usually state mileages as
liters of fuel per 100 kilometers (or something like that). In this case the
following formula would apply:

fuel economy (liters of fuel per 100 kilometer) = 100* fuel used (liters) /
distance drive (kilometers)

See
https://www.fueleconomy.gov/mpg/MPG.do?action=purchaseRecord
http://www.pege.org/fuel/convert.htm

If you want to get a true picture of your fuel mileage, you need to keep
records and calculate you mileage over several tank fulls of fuel (at least
three in my opinion) to minimize errors associated with filling the tank.

Regards,

Ed White

miles driven divided by fuel used.