Define "cruising". That is simply too general a term since cruising to one person is very different than cruising to another. If fuel consumption is really important to you I would suggest you install a fuel flow monitor which measures fuel flow and converts it to any number you want from gallons per hour, miles per gallon, fuel remaining, fuel used, fuel used since last fill, etc. The rule of thumb is 1/3 outbound, 1/3 return, and 1/3 reserve. If that's not a solution for you, it is quite simple to get a cruising fuel consumption reading. Fill up the tank. Add the number of people and gear you will usually have on board and go for a ride. Run at what YOU consider "cruising" speed (use GPS for speed and distance). Run 10 miles or so and then file up again. If it took five gallons of fuel you have the info you need. The answer is 2 MPG. The answer to the original question is 160 miles (80 gallons x 2 MPG = 160 miles). The problem is if you plan trips based on that one test and insist on stretching your range you WILL at some point run out of fuel. The best measure of fuel flow however is GPH (gallons per hour) but I won't get into that discussion because it has been discussed to death in this forum. Again, on bigger boats I suggest a fuel flow monitor.
