I'm a little stumped. My tach is off by 300-400 adter getting on plane. It idles at exactly where it is suppsed to (700). Idel speed is the same. Gets on plane at the same RPM it had been. No loss of power. 2 gps readings give out im still hitting max mphbut around 4.9 it's off. Max rpm (was 550o-5600) now im hitting 5200 ish at max. No loss of power or mph..the only thing ive done between outings is top off power trim/tilt (correct omc kind) .thoughts?

I should add if it matters the boat is a Ranger 482 and has 2 separate gas tanks on the port and starboard side more often than not I keep the same amount of gas in each and then use one to run down river and the other to run up river. Today I left one at about half or a little under half and I filled the one that was on The driver side fo The driver side full. What an unbalanced load with that much gas cause that? Probably not just a shot in the dark. The boat doesn't run any different and still idols and runs perfectly smooth so if I shouldn't worry about it let me know it's just something different to get used to. I use seafoam and startron run in every single tank of gas and always get ethanol free

Johnson 91' 175 fast strike.

I would suggest ordering one of those cheap tach's off ebay you just wrap the pickup around and plug wire and compare it's reading to yours.  I've seen props get an edge rolled on them that can drop the rpm but not the speed.

Your tach is probably out of calibration if it diesn't agree with the GPS readings, easy to fix at a tachometer service.

Balance is very important to top end speed. Your gasoline weighs 6.5 lb per gallon, your tackle can be heavier depending on what you have in the storage lockers. And livewells. Water is 8 lbs per gallon.

Another thing to consider is prop slippage, the rubber clutch hub bushing may be defective.

Tom