You running a "79 Johnson 150 horse?
Most likely expect between 2-2.5mpg (that thing is capable to suck 22 GALLONS PER MINUTE)
40gals = 80-100miles, less a third = 25-30 mile range (1/3 there, 1/3 back, 1/3 for incase of problems).
55gals = 35-45 mile range.
I'm not sure if the 225 horse will be a little or a lot worse or a little better.
All depends on what you intend to do with the boat!