Actually our 12 MPG is American gallons. On our last trip in February/March we drove 9802kms = 6090 miles and we used 471.466US gallons (all driven in the US) = 12.917 MPG. This is not from a readout but actual miles and actual US gallons tanked, starting with a full tank (not counted) and at the end of the trip filling up (included).
Methinks that should be pretty accurate.
Elly