I have a 40 HP Johnson on a 16/48 Wareagle. I don't have room for a bigger tank, I have the regular 6 gallon. My guess would be around 12 miles or so. I have fished Grenada all my life with a 6 gallon tank, and I have been all over the lake........from Choctaw to Bryant's and back over the years and I have never run out of gas. I am a little paranoid, though, lol. I carry a 3 gallon extra tank with me when I think I'm gonna be running long distances. You could use a GPS to see how many miles you run, and then immediately fill your tank back up and see how many gallons it used and do the math.