tach reading on V6 merc

[procomp]

Finally got my grandsport out with the 225 promax on it.
I'm hitting the rev limiter with a 28 offshore cut and a 29 srx but buddies iphone gps is only reading 80 mph.

Is there a way for the tach signal to be wrong? If not maybe his phone gps is not accurate. Next weekend I'll bring out another gps.

 

[skewby]

are you running 1.87 or 2 to 1 gears?

 

[procomp]

1:87.
I guess my question is, could the motor only be doing 5800 but reading 6800 and making the limiter kick in? Its a weird one.