- 1). Fill your boat up with gas and hit the water until you have run the tank nearly dry.
- 2). Check the fuel flow computer to determine its accuracy. It should tell you how many gallons were used on the last trip, and that number should be close to the actual tank size on your boat. If your boat did not come with a fuel flow computer, install one because it makes finding your cruising speed much easier.
- 3). Fill your boat up again and reset the flow computer.
- 4). Bring your boat up to the speed at which you normally prefer to cruise and make a note of your engine's rpm. This number will act as your baseline. Look at the flow meter's reading for how many miles you're getting per gallon. If your computer only displays gallons per hour, divide your boat speed -- its miles per hour -- by the gallons per hour reading to get the miles per gallon.
- 5). Increase the rpm speed by 100 by opening the throttle and keeping an eye on your tachometer. Make a note of your mpg reading once you hit the correct marker.
- 6). Increase the rpm speed by another 100 and record the mpg rating. Try a few speeds both above and below your baseline cruising rpm speed, adjusting by 100 rpm each time.
- 7). Look at each of the tachometer speeds you measured and compare fuel usage. For example, if you normally cruise at around 15 miles per hour, you may get 3 miles per gallon. When you increase by 100 rpm, you may notice your mileage goes up to 3.1, and when you decrease by 100 it could go down to 2.8. Whichever speed gets you the best gas mileage on the least amount of gas is your ideal cruising speed.