On Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

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?

The GPS is more accurate - the speedo is relative and usually works on
air - much like a pito tube in fact, only in this case, it's water
compressing air in the line. The only real true speedo, is the paddle
type as that works on wheel RPM and doesn't rely on air compression.

Having said that, there shouldn' t be a major difference - more than
10%. Usually, they are pretty close. If you have the air type, take
it off the guage and probe (or motor), blow some air through it and
put the line back. That should bring the two closer together.

You might also want to check if you have the probe type, so be sure
the probe is staying in the water and not moving around a lot - also
want to check the exposed part of the line - I've had a leak in the
line and a probe that moved around.

If it's the paddle type, then look it over and see if there is some
icky sticky stuff creating a problem.

Good luck.

Take care.

Tom

"The beatings will stop when morale improves."
E. Teach, 1717