They all seem to be off. Some more than others. My Donzi actually read low(we can't have that!) and I traced it to a crappy pickup. Replaced the cheap plastic pickup with a stainless unit and it then read about 4 mph high at WFO!

i have most of that but dont know the raito on trs drives and what is a good number for slip % ?

TRS ratio is 1.5:1, Just for a rough guess, I'd say with a TRS drive, you will have around 15% slip. (possibly higher) I've seen older TRS boats with the clamshell style water pickups mounted directly infront of the props. Audio's old 302 Formula (twin 330's) was spinning 27p props at 5000 rpm. His slip # was close to 30% !!!!

my stock Norskog speedo with a stainless steel pick up is almost dead on to my handheld GPS........if anything the speedo reads a little slower.

WFO my speedo reads 80, my gps shows the real speed (74 with a full load). in a small window that my passengers cant see.

On a poker run last year my passengers were telling everyone that we were running 80

My speedo is a VDO that is surprisingly accurate with the GPS. However it is the exception and not the rule.

Ive got a gaffrig speedo in my baja that reads almost dead nuts on to the gps. I wish it read higher than it does

MINE IS ALSO

my nordskog dead on ba**s accurate unfortunately, need to change the whole set of gauges thou, so the next one may say I run 75!!!

My Gaffrig speedo read high, right on, or low depending on the condition of the pickup tube. When I put a brand new one on it would read +5. After a while the pickup would end up getting bent and if it was at the right angle it would be right on or close. Then it would get bent more and read low.

I got tired of buying new pickup tubes. Got a GPS speedo, DONE!