You dont need the MFA to calculate the mpg! Simply fill the tank - right to the top. Drive around, and use up whatever fuel you want. Make of a note of the total mileage done on the full tank, then refill it to the same level - this is why it needs to be full, then you can see the fuel at the top of the filler neck. Make of the note of the litres you put in, then MPG is simply the miles you did divided by the litres x 0.2200. That will convert litres to gallons.
Easy as pie. Only difficult bit is filling it up to the same level, but its not too much of an issue, as a little bit wont make a lot of difference
