Garmin GPS 72 - Unusual readings ?

I recently purchased a Garmin GPS 72. Since it's not really boating weather
here - and the trailer isn't done yet - I've been experimenting with it in
other
situations. Last week I helped a friend of mine drive a Pensky box truck
with a car carrier trailer (and car) from Boston to LA. We took turns
driving
and sleeping and made it in 71 hours.

ANYWAY - I took the GPS along and got some strange readings. On the
trip page it shows average speed while moving, overall average speed, trip
odometer, etc. The other thing it shows is max speed. I've used this in my
car and it seems to work OK, but in the truck it sometimes showed
max-speeds as high as 115 mph. The truck *could not* go over 75.

I figured this was probably some interference from being in the metal
cab, from the engine (which is under the cab) or from the radio or our
cell phones. So I moved the GPS to a spot away from the engine and
cell phones and the problem seemed to go away.

BUT - in CA we went on a 6 mile hike/walk. Again I brought the GPS and
for a while it operated normally. But by the end of the trip it showed
max-speed
as 15 MPH. I know we were only walking at 3 or 4 mph. We never jogged
or fell or anything - there is no way we got up to even 10 mph. And I
didn't have
any cell phone with me to create any interference. Though I did have a
digital
camera (off) in the same bag.

So, questions:

- Is this normal?

- Why might it happen?
(Is it innaccuracies of the GPS system, this particular model, or maybe
interference?)


- If I see that the max-speed is inaccurate, should I also assume that
the other readings (odometer, average speed) are inaccurate?



Gary

 >
 > So, questions:
 >
 > - Is this normal?

Yes.

 >
 > - Why might it happen?
 > (Is it innaccuracies of the GPS system, this particular model, or maybe
 > interference?)

Speed is calculated by using simple time/distance calculations, measuring
the time it takes to get from one known fix to another. It is common for a
GPS to get an errant fix from time to time. Sometimes, these errant fixes
can be quite a ways off, but are usually only momentary. Nevertheless, they
are included in speed calculations. The seemingly unbelievable max speed
results from the very fast side trip your GPS thought you made while
visiting the errant fix. I've seen max speeds in the many hundreds of MPH.
Probably the closest I'll ever get to warp speed in my lifetime.


 >
 >
 > - If I see that the max-speed is inaccurate, should I also assume that
 > the other readings (odometer, average speed) are inaccurate?

Inaccurate is a relative term. I would suggest that your GPS is likely to
be more accurate and reliable for the above measurements than any other
instrumentation that you have on board.<!-- ~MESSAGE_AFTER~ -->