How Accurate Is a Stock SpeedO
 

Any one have any idea

Re: How Accurate Is a Stock SpeedO
 

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

My Livorsi speedo says 85, gps says 80

Re: How Accurate Is a Stock SpeedO
 

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.

Re: How Accurate Is a Stock SpeedO
 

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

Re: How Accurate Is a Stock SpeedO
 

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

Re: How Accurate Is a Stock SpeedO
 

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

Re: How Accurate Is a Stock SpeedO
 

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

Re: How Accurate Is a Stock SpeedO
 

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.

Re: How Accurate Is a Stock SpeedO
 

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

Re: How Accurate Is a Stock SpeedO
 

Here is a formula that most use to calculate speed.


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

Re: How Accurate Is a Stock SpeedO
 

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! :D

Re: How Accurate Is a Stock SpeedO
 

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

Re: How Accurate Is a Stock SpeedO
 

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% !!!!

Re: How Accurate Is a Stock SpeedO
 

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.

Re: How Accurate Is a Stock SpeedO
 

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 :D :cool:

Re: How Accurate Is a Stock SpeedO
 

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

Re: How Accurate Is a Stock SpeedO
 

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 :D

Re: How Accurate Is a Stock SpeedO
 

MINE IS ALSO

Re: How Accurate Is a Stock SpeedO
 

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!!!

Re: How Accurate Is a Stock SpeedO
 

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!