What kind of eng do you have ( I may have missed that?)
I have a 5.7 V/8 on my V 20 cuddy with a 4 barrell carb and I average 3.5+ mph at 3000rpm @ 30mph.
That equals out to a 60gal gas tank at 3.5miles per gal = 210 miles.
__________________
Tis better to be quiet and thought a fool than open your mouth and prove it!!
1991 V-20 cuddy I/O 350 volvo duo prop, 1998 15ft Grumman 9.9 Johnson