There's loads of variables doing a road test for something like this, in addition to the inconsistencies of the test driver/drivers.That will also work, but far more realistic fuel consumption figures can be obtained via a road test as I described above.
A dyno will be more for tuning any kind of engine for maximum power and low fuel consumption.
Besides, if you really want to lower your fuel consumption drastically, get a scooter/motorcycle, or use a bicycle![]()