trying to figure out the speed of my boat it pegs out at 75 in my 311 with 525hp sets on the vdo emblem.

I would say its possible but you would have to be turning them over 5600rpms (guess of top of my head) and have pretty low slip #. What rpm you turning? I get 75-76 at 52-5300 rpms non labbed 25's in a smaller boat.

5200rpm with 25s and it will go way past the 75 on the speedo

If you can spin the 25's at 6000, have a real slip factor of 15%, you would in theory run 80.492423 MPH !!! It's all in the math.

Only sure way is to GPS it, mechanical speedo's can vary.

My stock 26 Bravo 4 blades go 72-73 at 5250 on a 311 with 1.5 Ratio Bravos.

1.3 gears in the drives, then no problem.

A 1986 VDO speedo is probably about as accurate as a $2 watch.
You need to get a GPS speed.

1992 Formula 336, Mercury Mirage Plus 25" props, slightly built HP 500s. Vdo speeds 76 to 80 mph. Gps speeds 70 to 72.5

13% slip (a good # for that hull when set up well) would require 5826rpm to get 80mph

Brett