You cannot really work out distance per gallon on anything such as a boat as you will affected by winds and tides. Best you can calculate is available engine run time for a given amount of fuel.
See if you can find data on your particular model engine, from that find out it's fuel consumption per hr @ xxx revs. Usually running your motor at about 75% power gives (if it's a diesel) will give the optimum for speed/consumption ratio.
There's other variables to consider as well. How clean is the hull, how efficient is your prop.
Try to get a ballpark figure of the per hr consumption of your motor at optimum revs. Then do several "legs" and see what log speed you are getting.
Once you've got that add 10% for a safety margin.