For feck's sake Valley Boy, how difficult can it be?
Zero the trip meter.
Brim the tank.
Ride the fecking thing, with or without a pillion.
Brim the tank again.
Take a note of the mileage on the trip meter and the number of litres pumped in to brim the tank.
You will now have a rough idea of the number of miles you have covered per litre.
Repeat the process to give you a better sample.
After say three or four calculations you now have your average miles per litre. Most bikes do about 10 miles per litre as a rough rule of thumb, to give you a vague idea.
There is a website that will convert miles per litre into MPG if you struggle with the sums.
If you are getting fifty or more miles per gallon, then you or your Tripple Black (I have to guess) are either calculating it incorrectly or you are a flower presser of the first order. If the latter, we salute you.
Now, what numbers do you get?