Speed on PP vs. speedo

Installed Perfect Pass today - works great - recommend it to anyone. However, the regular speedo reads about 3 mph slower than the PP display. Which to trust???

Neither! Get hold of a GPS and set the PP. Then set the speedo to the PP. Chances are that both of them are off.

If you use a GPS, now you have 3 different speed readings. How do you know that the GPS is that much more accurate than either the PP or speedo? I have heard that recommended before so I was curious.

Actually when I first got my PP the speed was off by about 3.5 MPH. It was reading high (ie. PP 18 speedo 21.5). Used GPS to calibrate and i believe it is accurate, you certainly have to trust your GPS

GPS won't be perfectly accurate unless the water is perfectly still. However, I don't know of any other way you could calibrate it, and thats what PP recommends to use. When my PP was installed, it was way off. We set it at 20 just to test, and the GPS was reading 26.8, or something close to that. I would calibrate it in a lake, and not the delta or any other river system. You could also try running next to someone else who has PP and see what the difference between your speedos is.

mb

The Speedo and PP were pretty close when I purchased the boat. I got up one morning very early and went out on the glass and tuned both dead on with my GPS. I have not had to mess with the settings after 40 hours of use but check with GPS anyway every few times I go out. It is funny when people are use to riding in a boat that has nto been calibated correctly. People tell me what speed they want and it is either faster or slower than they are use to. BTW, I hardly eve look at the Speedo anymore as the PP is so easy to read.

Reminds me of the saying: "A man with one watch always knows what time it is, a man with two watches is never really sure".

No two speedos are ever going to agree 100%. Pick one and use it all the time. I recommend the PP.

Jessica, on a lake (versus a river with a current) a GPS is extremely accurate since it is constantly calculating your current position based on a highly accurate timestamp received from several satellites (up to 12 with newer GPS receivers). Since speed is easily calculated by dividing the distance traveled by time moving, the GPS will always be highly accurate.

So it's fairly easy to calibrate PP to the GPS, and then the speedos to the PP. Even after this is done there may be differences at different speeds. For example, if you calibrate PP to the GPS at 22mph, at 40mph PP may be off by 1 or 2 mph. For that reason, it's recommended that you calibrate PP at the speed you normally board.

GPS "can" be very accurate. They can also be off enough to matter for this application. Depending upon the exact location of the satelites, how long your receiver has been on, and if "selective availabilty" is turned on or off you can get a significant amount of error. I have seen GPS units indicate a speed of several MPH while the boat was tied to the dock!

You should be able to get a reasonable sanity check, however. Start with the boat at rest. If the GPS says you are going 0.0 MPH, then a speed check should be viable. If it reads anything else, wait 'till the satelites are in a better position.

Wait till the satellites are in a better position?!?!? Huh? I thought GPS satellites were stationary, and the signal quality depends on how many GPS satellites your receiver can pick up. GPS can be very accurate if you have enough satellites to pick the transmission from. This can vary because of terrain, weather, etc.

Trash

Has "selective availability" ever been turned back on since they turned it off several years ago? I assume that a boat in the middle of a lake would have a clear view of the sky and could easily acquire enough satellites for an accurate reading.

If my GPS was reading a couple of MPH while I was parked at a dock I'd assume it was time to buy a new GPS.