NateN wrote:
Just got a new GPS and tested my speedometer with it. I found that it
is off by as much as 4 mph, but only at slower speeds. For Example,
when the boat speedometer reads 5 mph, the GPS shows 8.4 mph. At 20
mph or so, it's pretty accurate.

Is this fairly common?

First thing, those speedometers are not very accurate. But another
thing to consider, those units measure speed over the water, a GPS
measures speed over the groung. If you were in a current, moving into
the current, the speed of the current is added to the speed over the
ground, so the unit will read higher than the GPS. The reverse applies
if you are moving with a current.

I once got a speeding ticket for exceeding the 5 MPH limit, the cop got
me on radar at like 8 mph. The radar gun looks at the background in
referance to your movement, so it gives speed over ground. I took it to
court and fought it. I got local current charts that indicated the
current was running at over 5 MPH at the time in that channel (a flood
tide, a real good one). I was lucky, I had a judge that knew something
about boating, he threw the ticket out and had a little "chat" with the
officer who was obviously clueless about what we were talking about and
knew little about the problems associated with slow speed operation
when running with a current.