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# GPS or Speedometer

Vanman
0 Points

At highway speeds my Nuvi 760 conistently reads 5 MPH slower than my speedometer...which is more accurate? But, really officer, my GPS had me going just 65 :roll:

## Comments

original size tiresthat came with your car?In fact, I used to do custom tuning on a truck I had and used that to dial it in.

http://www.news.com.au/heraldsun/story/0,21985,21999706-661,00.html

With 24 orbiting satellites using triangulation, and GPS speed being calculated using an atomic clock, not some old worn out treads on those tires … Guys, forget about speedos on your car … Only laser guns can claim to compete with GPS accuracy and even them have lost in court.

Cheers!

HappyTrails! Drive Safe!

Cheers!

HappyTrails! Drive Safe!

I've clocked my speed using:

My speedometer

Previous Lowrance iWay 350c

eTrex Camo

gps12

Previous nuvi 350

Previous nuvi 360

Current nuvi 660

Current Freedom Keychain GPS through my Motorola Q

The only one that gave a different reading was my speedometer.

Not saying your findings are inaccurate, just posting a different result.

What brands did you compare?

Something else probably related is to look at the speed your GPS shows while it is "at rest". Now many of the auto devices are likely programmed to show 0 when it is close to 0. But for example I just turned on my 760 and have left it in place for the last five minutes and it shows an average speed of 1.7 mph with a max speed of 3.1 mph. That is likely a good indication of how the margin of error of GPS impacts the speed readout.

I'm surprised that your speedmeter showed a lesser number, I would have expected the opposite. It's a known fact that most car manufacturers have cranked up their speedos for a number of reasons, lawsuits for one. I think it's fair to say that that dial is an indicator, NOT a precise measurement; such that if you're showing 110 mph, odds are that you're probably doing only 109 or 108 or 107 - the variance can be as much as 5 mph depending on the manufacturers. There are some exceptions, the 2007 Toyota Camry for example is dead on - why this one and not others! It beats me!

Cheers!

HappyTrails! Drive Safe!

0.00 mph max speed

/hijack 8)

Cheers!

Actually GPS calculates instantaneous speed. Position is calculated based on phase of received signal (pseudorange) and speed is calculated the same way based on Doppler frequency of received signals.

So if the position is off by 10 meters behind your actual position one second then 10 meters ahead of your position the next second you have an error of 20 meters over a one second gap. That will easily account for speed calculation errors regardless of terrain.

As to vertical travel, a little Pythagorean Theorem math will show that it is irrelevant to the calculation, even in mountainous driving, given the horizontal errors already mentioned.

In more critical applications (military) there may be an advantage to using the Doppler returns along with the base derived position data and combining these measurements in a more sophisticated filtering scheme (i.e, Kalman filtering).

Here is a link to a discussion of this topic (this "speed calculation" question seems to pop every year or so) that is hopefully more informative.

http://www.gps-forums.net/speed-calculated-nuvi-660-a-t42724.html

Somehow I think that idea of computing speed form successive position appears as a best guess form people surprised by the question. Not that I know GPS algorithms, but after reading some more detailed description of how GPS works, I tend to assume that Doppler frequency is the obvious way to compute speed.

I still think that commercial GPS's simply take 4 measurements (per sample time) in order to get a set of non-linear equations where the unknowns are the vehicle position components and a time bias - satellite positions are known from downloaded data, typically in an Earth Centered Earth Fixed (ECEF) coordinate frame. These equations can be iteratively solved for the desired unknown variables using simple, established numerical methods. Velocity data is then obtained as a filtered derivative.

An alternate (non-Doppler) way to get the velocity components would be to set up the problem by differentiating the position equations to obtain a set of non-linear eqs. in the unknown vehicle velocity components (using the position components previously obtained). This results in a set of equations that use the angle information to each satellite (direction cosines) and the known satellite velocities and positions to iteratively solve for the velocity components. A lot more complicated that a filtered derivative.

Anyway, just my best guess of what is actually being performed.

I consider GPS not very reliable when it comes to speed readings. Heck, I went for a hike over the weekend and it said my max speed was over 22,000 mph with a few other times when I was hiking over 70mph. :)

Another interesting observation is that I have an UltraGauge device on my Hyundai Elantra. It is an OBD2 connected device that monitors many different data elements on the car's electronic control system. The UltraGuage speed matches my Garmin speed very closely. And since it has access to the same data as the car speedometer, it makes me think that Hyundai calibrates the speedometer to show fast on purpose.

standard.I've used my Nuvi 755t in several cars.It never quite matches the car speedo,its either higher or lower.Thats because every car speedo is a bit different from the other.With all the tech. involved I trust my GPS more than speedos in cars.

That's just what I think happens, nothing at all scientific or measured about it.

I do know that I trust my GPS receiver's speed reading more than my speedometer's, based (as another post earlier) on passing "Your speed is" radar signs, as well as timed passes through mile markers on Interstates.

I experienced this frequently while mountain biking off road. If I was riding on a relatively straight road, the GPS and my bike computer would record distance pretty comparably. BUT, if I was off road on switchback trails, the GPS would always show less distance traveled than the bike computer did. I rationalized this as the GPS measuring a straight line distance between it's update points, whereas the computer recorded actual distance around the curves.

In marine charting, it's not unusual for the position fix to show a docked boat jumping from one side of the dock to the other every second or so based on accuracy errors.

We also experienced the same effect tracking Iditarod dog sled competitors. The actual trail distance as measured by snow machine odometers was always greater than their GPS distance. For that reason (among many others) no one can tell you the actual distance of the race.

If you use a non-routable topo map, then road lock will not be supported and you should get more accurate results.