Speedometer Accuracy
 

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?

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.

"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.

"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.)

Around 8/11/2005 8:56 AM, 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?

I wouldn't know; my pitot speedometer is scaled in MPH, but at speed in
still water it seems to more accurately reflect my GPS's *Kt/h* reading. :)


--
~/Garth - 1966 Glastron V-142 Skiflite: "Blue-Boat"
"There is nothing - absolutely nothing - half so much worth doing
as simply messing about in boats."
-Kenneth Grahame, The Wind in the Willows

Thanks to everyone. Sounds like it's behaving somewhat normal. Now
that I know how it acts, It's not going to be a problem. At least I
won't spend time arguing with a boat dealer or mechanic over it.