Look at it this way. The antique outboards of the 1930s came from the factory with a built in fuel tank that would run the motor about an hour at wide open throttle. (This varies by boat design and load.) A 5.4 horse Zephyr held 7/8 of a gallon, a 9.7 horse Lightfour held 1-1/4 gallons, a 22 horse Speeditwin held 2-1/-2 gallons, a 33 horse Speedifour held 4 gallons. (Same tank was used on the 50 HP Big Four & Racing 460) Our Big Four got about an hour and fifteen minutes on a tank. Plotting a course around the lake, I was getting a little over 13miles to the gallon.

I recall getting around 3 hours on our 25 horse Big Twin. I felt that that was about right. We had three tanks, two in the boat and one on the dock. We usually ran 3 or 4 hours after breakfast, then stop for lunch, then wait an hour to let lunch settle and then go out again. If the weather was nice, we came in when it started to get dark.

Dad had gotten two telephone poles. one was cut in half and set in concrete with about 5 feet sticking out. The other pole was mounted between them and held in place with a couple lengths of 1-1/4" pipe. A ten foot 2X4 was bolted to a notch five feet down from top of the pole and four lights were mounted on it forming a square. A flood light was mounted about half way down to aluminate the dock. When lit at night it looked from the lake like a bright light over a dim light. We could always find our way home . . . 😆

I’ve owned and ran a number of 25-40 hp motors back in "The Day", fishing, cruising, skiing, whatever. As best as I remember (??), they will run a bit less than 2 hours on a tankfull, while skiing. Hey, a 50hp could suck up about 8 GPH.

When you run at 3/4 throttle, it’s important to push it to wide open and then back off to 3/4 – this backs off the carb but not the spark advance. The owner’s manual refers to this as "Cruising speed". The 40s from back then worked the same way. If you just run it up to 3/4 without going to WOT first you will use more gas.

If you get the newer design 35 HP, beginning in 1976, you will have better mileage, but the low end torque is not there for planeing the boat and pulling up skiers, especially noticeable on heavy boats like most wood and glass ones. These motors have smaller cylinders.

Like the others said, be sure you have the optimum propeller pitch for your use and load.

Back in the day on Dad’s old 14′ aluminum Crestliner, we used more gas running a 1956 30 HP and later a 1960 40 HP than when the 40 was replaced with a 1965 V4-60 HP. The 30 and 40 were running at WOT almost all the time pulling skiers, while the 60 loafed along at about 2/3 throttle doing the same thing. The V4 60 was heavy for the boat and screwed up handling some, so it was no longer a boat we could let anyone drive.
Dave

Here are some more prop options at reasonable prices, but still not an 11 inch:

http://propcopropellers.com/evinrude—johnson.html

Wood boats are generally heavy compared to Aluminum and require a lower pitch prop for optimum performance

1 inch makes a big difference in performance, so 1/2 inch will be noticable. Note that as pitch goes up (unless it goes too high), top speed goes up, RPM comes down, but low end planeing and pulling up skiers and tubers suffers.

I recommend you find what RPM you are running at at wide open throttle now, compare that to the specified operating RPM that’s on the HP rating ID plate. You want to stay within that recommended range. There are portable tune up tachs that can be temporarily connected by clipping a wire around the outside of a spark plug wire without making a direct electrical connection.

Another suggestion – have you tried all the positions of the trim pin? One notch higher on the trim pin can get a lot more of the boat up and out of the water, reducing drag and increasing fuel economy. Run the pin as high as you can without the boat bouncing while going straight or cavitating (losing it’s grip and revving up) on corners. Having as much weight at the rear as possible also lets the boat ride higher, so if you r gas tank is under the front deck, consider moving it to the rear.
Dave