Your gps will be absolutely accurate (within 0.10 mph)
Your boat speedo will be +/- 10% usually but some are worse than that.
The boat speedo is just a hit/miss on being right. At some speed they are
usually right but all other speeds they will be off.

--
Tony
my boats at http://t.thomas.home.mchsi.com

"JGK" wrote in message
. ..
Just curious as to the accuracy between the two.
The speedometer in my boat is a standard speedo nothing special, the
GPS is a Hummingbird Matrix 25 which I just got a week or two ago.
After a few outings I noticed that there is at times a considerable
difference
between the two and other times they seem spot on. Can't quite figure
that
one out. Would a head current account for the boats speedometer to appear
at a higher MPH than the GPS?