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WGA Board Member
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Joined: 2005-01-17
Posts: 2558
Location: Mountain, WI

PostPosted: Fri Sep 16, 2005 9:55 pm Reply with quote Back to top

how accurate is the Current Speed reading on your GPS our Garmin 72 is showing slower than our car is.
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WGA Member

Joined: 2005-05-11
Posts: 210
Location: WI, USA

PostPosted: Sun Sep 18, 2005 1:40 am Reply with quote Back to top

Most speedometers are calibrated to read a bit higher than you're actually traveling at. Saves you tickets and them lawsuits over tickets. Interestingly, Harley Davidson motorcycles tend to be accurate at all speeds.
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Joined: 2005-05-11
Posts: 210
Location: WI, USA

PostPosted: Sun Sep 18, 2005 9:18 am Reply with quote Back to top

I can't believe nobody gave you a detailed answer yet, for as many "GPS nuts" as there are out there. So I took some time to research the answer: It depends.

If you have perfect conditions, i.e. no clouds or other atmospheric disturbances, WAAS is locked and all 12 satellites are tracking with full signal, you're moving at a constant rate of speed at a decent rate of speed on flat terrain in a straight line, etc., your unit should be accurate to about 1/10 of a mile per hour.

The GPS updates the readout every one second and averages your speed over several readings, so you can't expect 1/10 MPH accuracy, since you won't always be going exactly the same speed. Also, the faster you go the more accurate the reading, partly due to the "resolution" of the receiver. Your GPS is probably sitting on your dash with 1/2 the satellites out of clear view due to the roof of your vehicle, also decreasing accuracy. Although the GPS will try to account for elevation changes, the elevation accuracy doesn't seem to be nearly as good as the surface accuracy, although at this point we're getting pretty picky.

There are many other factors, but the bottom line for most any GPS is: If you're travelling at a fairly constant rate in a straight line at three or more miles per hour with decent reception, the accuracy should be within 1/2 MPH, far more accurate than your speedometer.

If you're wondering, the average reading for speedometers used to be 25 at 23 actual, and 60 at 55 actual, but I think that newer speedometers often read 58 at 55. I base this on gut instinct from reading hundreds of reports over the years, as I used to restore motorcycles as a hobby and read tons of reviews of both cars and motorcycles. Yes, I have 5 bikes in the garage now. No, I can't ride them all at once. I have a lot of different foods in my fridge, and I don't eat them all at once either.

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Joined: 2002-02-19
Posts: 3251
Location: Milwaukee, WI

PostPosted: Sun Sep 18, 2005 9:45 am Reply with quote Back to top

Great info Opossum.

The speedometer in every vehicle I have checked with a GPS has read low or right on, usually 3 to 5 mph low. Some people think there is a law that requires speedometers to be optimistic, but I think it is just the manufacturers covering themselves. But there is such a law in Germany, and to make sure they are in compliance, the speedometer on my BMW motorcycle reads 6 to 8 mph higher than actual.

I have used a GPS in a police vehicle with a calibrated speedometer and both gave the same speed. I have also had my speed "checked" by both radar and laser and found the GPS speed was right on.

So now that you know your speedometer is actually reading higher than you thought, you can drive a little faster (and therefore find a couple more caches in a day). Image

[This message has been edited by jvechinski (edited 09-18-2005).]
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Joined: 2004-05-06
Posts: 2008
Location: Waukesha, WI, US

PostPosted: Sun Sep 18, 2005 4:47 pm Reply with quote Back to top

I mave a Magellin SporTrak Pro and it's almost on with hubby's car. Found that out when we were coming home from Kansas and looking for a close cache. Looked down and saw he was doing 78 in a 55 LOL Looked at the speedometer and it was pretty close. He wanted to know HOW I knew how fast he was going since I was saring at my GPS.. Told him I'm psychic!!
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