"NateN" wrote in message
oups.com...
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?

Not only common, but unavoidable.

A Speedometer (or log) measure the boats speed thru the
water, while the GPS calculates the speed over ground by
measuring how long time you used to move between two
coordinate positions.

So your Log is influenced by current and mechanical inaccuracies.
The GPS is influenced by position accuracy, which it nulls out
over time using averages.

If your log is a good one, and it have been calibrated properly,
the difference between the GPS and the log is quite useful.
Together with your compass, you can tell the strength and
direction of the current.

With that information, you can correct your course for drift
caused by the current, and plot a straighter course.

--
Dag.