The three gal tank will be good enough. But suggest you take a extra can of gas. It can be another 3 gal. can or more. This is for safety. Measuring the gas consumption on a 7hp engine is more difficult because of so many variables. It will run all day 8 to 12 hours at trolling speed on 3 gal of gas. But at full speed this will change drastically. Distance traveled isn't a very good way to measure gas consumption on a engine used on water. Looking at your watch and seeing how long it takes to burn 1 gal of gas at full speed is the best way. It's called "Gas Per Hour" (GPH) and all larger boats should have this meter on their boat. With this knowledge you can calculate the actual gas that will be consumed. It's more accurate than a gas gage which are often not reading correctly.
Good Luck;
PS; If you was traveling on a river with the current against you it is possible to travel and not go any miles at all and burn all your gas.
Good Luck;
PS; If you was traveling on a river with the current against you it is possible to travel and not go any miles at all and burn all your gas.
