Calibrating speedometer

[Searay]

On a 1996 SN GT-40, with a 4 blade prop, what is the common way to check if to see if your speedos are reading close? A friend of mine says they should read 36 MPH at 3600 RPM. Does this sound right? Thanks.

[Rick]

It sepends on the pitch and diameter. The best way to check it is with a GPS system. As a rule of thumb with a 12.5X15.5 prop your friend is right.

[SilverSpraySports]

Your best two bets are to either time it in a slalom course or get a GPS. The GPS is probably the easiest way. On a 1995 SN with a gt-40 and a four blade OJ prop, we run 3650 for 34 mph and 3850 for 36 mph. These come out with a pretty good time in the course.
Hope that helps.
-Doug