You can do a test ride, just ride a whole tank only
driving up to 90km/h in the highest gear. It will take some discipline.
Not many riders are willing to do the test.
But, and this important, only than you will get an idea of how low the
fuel consumption of your CB can be.
On the other hand it is afterwards very easy to push the consumption into unknown heights.
Wisedrum, mostly be a testrider