"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?

Your speedometer will read down to 5MPH? (Mine won't budge at anything below
about 10-15 indicated.)