I recently bght a 86 242ls with a rebuilt 454. Boat is great. The stock speedometer is reading about 47mpm at full speed. Boat seems like it is going much faster. How acurate are these stock speedometers?

Not very. Really need to get a GPS or borrow one to see how fast you really are going

I have a 1990 242ls and suprisingly my spedo has proven accurate when compared to GPS. However, speedos are usually inaccurate.

To give you a point of reference, I'm running 56mph on GPS with a 365hp 454 mag spinning a 23p three-blade Mirage (not plus).

Ok great. I am not sure what prop i have but at full speed my tach reads 4000rpm. Any advice on a different prop to get more top end?

I know of a couple of 242s in the area. Ones a 365 mag and runs 56mph another had a 502 mag(415 hp) and best was 61 with a labbed 24" bravo. It now has a warmed over HP500 @ 535 hp and has seen 69/70 mph with the same labbed 24 @5400 rpm. Going to try a 26 lab next time out