Is it possible to adjust Yamaha speedometer?

[fshmn]

My Yamaha digital speedometer reads 10-20% less than my gps speed.

Since I am in the process of installing a fuel management gauge, if I use the speedometer input, my fuel mpg readings will be incorrect.

I can use my gps to provide speed input to the FM gauge, but, I would prefer to use the speedometer input if I can adjust the output to be more correct.

Thanks.

[boats.net]

No there is no adjustment just check speedo tube and pickup for restrictions like plugged hole or kinked hose.

[99yam40]

If you are running with a current or against a current there will be a difference between the GPS and speedo, but it would a have to be a strong current to get 10-20 percent depending on the speed you are running.