Is it safe to assume that any device that gets speed readings from satellites (G.P.S.) would be the same. Or could one model chart plotter be off a certain percentage from another? Basically if you made changes to affect the speed of your boat should you always use the same G.P.S. device?.....Thanks.

Every GPS should be within 0.1mph of each other. Some might have slower update rates, but they are all using the same satellites (when operating in the same area).

i'm gonna make a gps that reads 10 mph high. it will sell like hotcakes. cheapest 10 mph speed addition you can buy!!!

A guy who used to post here and on speedwake all the time got about a 10-15 mph GPS spike on his 32 foot Sunsation when doing a speedrun once so wars were argued and fought over his true top speed. When I take my boat out and make actual speed or testing runs I ALWAYS put a handheld with re-call that is zero'ed out in my glove box to compare to the re-call on my dash gps, shuts up the nay-sayers- they usually read within .2 of each other, on my best speed run the handheld read 99.7 and the dash read 99.5, I have had my gaffrig dash gps spike and read 10+ mph high, my garmin handheld nevers seems to spike though.
To answer your question-if your doing something you consider important use 2 of them in case one malfuctions and to back each other up, FWIW, Smitty

Thanks everyone for the info. because I had my props tweaked, and my old G.P.S. died ,so I replaced it and wasn't sure if I could compare apples to apples from the old one within or less than 1 mph accuracy.

i been using my i phone with a gps downloaded lately anyone know how accurate that is?

Funny this topic should come up because just this Thursday I went out for a ride on my boat with a friend. This was the 1st time in a while I used my Garmin 640 on the gauge screen which shows the speed. I also have a GPS speedo on my dash that I always go by......while running I told my friend we hit 80mph, he said not according to the Garmin, we were at least 8-9 mph slower. I was upset and worried my GPS Gauge was not accurate (I was panicing). I then quickly opened up the iPhone app "Speedo" and did another run. It was dead on with my GPS Gauge on my dash while the Garmin was still way off. I then looked close at the Garmin myself to realize it was in knots....once I swithced it to MPH all three units were dead on. So my answer to the question about the iPhone being accurate is yes.......It was dead on with my other GPS readings.....

I think the iPhone accuracy is very dependent on the cell signal you have in the area. I have noticed my iPhone is pretty accurate, but I boat on the Detroit river and cell signal is great. Since the iPhone uses cell towers instead of sattelite it would make sense the accuracy would be based on your signal.... I am not a gps or iPhone expert, so this is just my theory.

very true......

If the iPhone is using cell towers then its triangulating based on ERP and know cell tower position. This is not nearly as accurate as GPS. The best method is for the device to use GPS signals only.

Also keep in mind that GPS is good for steady state and is horrible for acceleration.