Try the online miles per gallon calculator tool

Mpg Calculator
for miles from litres

Enter Mileage and Fuel used since last fill-up:

Distance  covered = miles
Litres of fuel used  =  litres   

Consumption:   Miles per Gallon
                          Litres per 100km    
                          Kilometres per litre    

The gallons are Imperial Gallons - not U.S.
(1 Imp Gal = 1.201 US Gal = 4.546 litres)