Any one have any idea

Its not accurate. But, then again, neither is GPS. Thats gonna start something here..

My Livorsi speedo says 85, gps says 80

Believe the GPS. Never believe a stock speedo. They are always fast. There are other variables with a GPS and maybe radar is the most accurate but GPS is the best we have to install in our boats.

I find my speedo is very accurate..within 1mph of my gps.

My stock speedo is 8 mph off at 2,500 rpms and 4 mph off at 5,000 rpms. too high!!!!

They are all off at various points. Each boat I have owned has been correct or off at different speeds. No rhyme or reason!

I call the stock speedo's passenger thrillers, my last boat did 10 over at WOT. My 22 is very close at all speeds which still surprises me.

Phil

Most speedos are very optimistic. However, I have a customer that I built an engine for that went from 58 to 73 on GPS. However his stock speedo read about 5 MPH slow. Man, that was just not acceptable! So, we installed a GPS analog speedo. That is the only stock speedo that I've ever seen read slow.

I didnt mean to start anything bad here... lol... but it sure seems that i am going alot faster than 60 in my scarab

Here is a formula that most use to calculate speed.


((pitch x rpm) / ratio) / 1056 = theoretical speed. Then you must allow for your slip %.