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MPG for a 25hp early 90s Yamaha

What RPM and boat speed?

What is the weight of the boat?

Flat bottom boat or V hull?

What propeller brand/model/pitch is installed?

What temperature/air pressure/humidity conditions?

Best case situation it would be something like this
2yl6yix.jpg
 
You measure engine performance in GPH (gallons per hour) for boat engines. You get the best economy by running it at max spark advance usually around 3/4 throttle.
 
You should get around 15 mph with a fiberglass semi V hull with 25hp in ideal conditions and drink around 6 gallons in 1 1/2-2 hours at WOT. Is the engine reaching proper WOT rpm? Is the hull clean? Is the double bottom dry? Is it trimmed properly and the weight distributed for a good plane? Which way is the water current and wind flowing? The only real way to answer that question is on the water and get to know your setup.
 
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