Well let's see, you say your using a "standard speedo nothing special".
Here's a thought, Buy a pair of competition Tow Boat Speedometer's as they
use for exact speeds that are critical for tournament skiing and then
compare the speeds with your gps. You still would have a difference between
boat speed and GPS speed. I agree with all the other posts in the GPS
technical side as well. I went to FAA GPS faqs and the below statement
should help with considering the above mention delay's

GPS FAQ's from: http://gps.faa.gov/FAQ/index.htm

" The receiver uses the time difference between the time of signal reception
and the broadcast time to compute the distance, or range, from the receiver
to the satellite. The receiver must account for propagation delays, or
decreases in the signal's speed caused by the ionosphere and the
troposphere."

Regards,
Ron



"Matt Lang" wrote in message
om...
JohnC wrote in message
. ..
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?

Yes, the speedometer is relative speed to water, GPS is actual speed
over surface of earth.

correct and add to it that boat speedometers are often just speed
estimators ... not very precise ..

Matt