I just got a new GPS which can show speed and distance. I tried it out in the car and it seems to be 4-5 MPH slower than the speedo and any given speed. So I'm assuming my speedo is off by a little because I belive GPS are considered very accurate at driving speeds. Maybe the needle was positioned slightly wrong? What do you think?

 

We had GPS on all our ambulances. GPS is very accurate. Th problem comes that cicillian systems are not allowed to be as accurate as military recievers. As great as GPS is, and Im a big fan........sometimes, on occasion, we had an ambulance doing over a hundred miles an hour while at a stop light. It happens.
If your reciever tells you all the time that your speedometer is off, you speedometer is off.

 

Does speed shown on a GPS unit take into account change in elevation?

 

Cowboy....
Please enlighten me on how elevation would have anything to do with the speed of horizontal movement. .
I have a Garmin E-Trex and it reads the same speed as the truck speedo. I would think the GPS could be checked by timing the time it takes to drive one mile, between mile markers at 60 MPH. and comparing it with the distance on the GPS. That could also check your speedo.

 

There is no more civilian and government differences. The false readings were taken away about a year ago.

My Garmin 12 is very accurate, and since my speedo and odometer are off by about 7%, I have figured out where the new speeds should be.

*mine updates twice every second.

 

It depends on the GPS receiver. The military controls all the satellites, and have a "correction factor" built into the system. The altitude reporting is where the "correction" is built in. Probably the reason you are seeing differences is because most of the civilian handheld GPS units are not designed to update often enough to be accurate at higher speeds. They are mostly designed to be used by hikers, slow movers, etc. I wouldn't count on GPS to check the speedometer. On the other hand, your speedometer could be wrong!

 

My Garmin 195 is very accurate. One way of determining how good a signal your unit is receiving is by looking at how many satellites it is receiving. Last week mine was picking up at least 6 (closer to 8 I think). I trust my Garmin over my speedo or the Loran in the plane.

The speed issue is non factor. They work the same at 1 mph or at 200 mph, etc. My aviation version works at any speed. Just give it a signal and you have a reading.

 

I think I am going to do some more checking on this. I know that GPS is used to survey points down to centimeters, but the surveyors need to download correction factors to be accurate. I haven't been involved in this for about a year, but I suspect that the correction factors are still in place. Especially with all the stuff happening since 9/11, it would indeed be amazing to me that they would get rid of the corrections. But, as Dennis Miller always says, "That's my opinion, but I could be wrong!"

 

Imagine you were in an airplane looking down at two vehicles each going 100MPH as reported on their speedos and they were both going straight down a road. Now imagine one was traveling on a dead flat road and another was going up a 45 degree slope (yeah, steep I know, but I'm trying to illustrate a point). As seen from the sky, which one would appear to be going faster?

With this in mind, I do wonder if GPS units account for this. I would hope they do.

 

Speed up or down hill would be irrelevant in a ground vehicle. Considering the most we will see at highway speeds is usually 6%, the correction for horizontal movement would be too minor to consider. It would be interesting to see how it works in an air plane though.

I am a Land Surveyor, and use both surveying, and hand held units. The military still only allows the public so much accuracy. On occasion, they allowed for a little more, recently allowing your hand held units up to 3 meters.

In the Surveying industry, we use 2 receivers to gain our higher rate of accuracy. One is a mobile unit we can carry on our back. One is a stationary unit set up on a "known position." With both these units reading satellites and the relationship between each other, we can triangulate our position to centimeters.

Some other facts you might not know..........

GPS is a very week signal. It does not work indoors, tree cover can even effect your signal. This can lead to difficulty in vehicles, around buildings, and even from standing over the antenna.

Most satellites will be found in the southern hemisphere. So if you are having trouble getting a signal, try to find a clear southern horizon. There are also web sites you can visit that will show you satellite availability throughout the day, so you can find the best time to take an accurate reading.

Oh ya, and I found my handheld Garmin III+ to be extremely accurate for speed.

 

GPS does determine altitude also- So vertical speed is factored in.

 

I have a Garmin Street Pilot 3 GPS and i love it. It, i belive, is very accurate, with good reception that is. I usually have between 8 and 10 satelites as reception and an EPE of 15-20 feet. Thats close enough for me and driving on roads I hardly look at my trucks spedo anymore, just the GPS.

 

I get a signal inside my house.... I can turn it on and it will be accurate to within 34 feet inside the house, and 8 feet outside.

 

I had a Garmin and what I found is the GPS appears "slow", especially on long down grades. It can be off on a very long trip (2400+ miles) by over 100 miles too. Your speedo will be a little more accurate on distance moved by the car than the GPS. The GPS is accurate on point to point, but, does not account for the slight differences in going up and down grades to get to that point. I imagine boating it will be 100% accurate.

I like the post about viewing it from a plane.

That being said, I would trust the GPS! Most older cars with gear driven speedometers come with a "fudge" factor. Even tire inflation can effect your speed. We use to bring cars or speedo heads to have the car calibrated at a speedo place. You would be surprised how much off a stock car can be.

 

a speedometer measures "instantaneous" speed
whereas a gps measures your average speed over the last few seconds, and in a straight line.
So any change in speed or change in direction will affect the speed reading on your GPS The only time you can get a true speed reading on a GPS is at a connstant speed and a straight line.