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Old 09-06-2003, 05:17 PM   #1
brunetmj
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Default Speedometer versus GPS

From previous threads , and from my own experience, the speedometer does not match the vechicle speed as shown by a GPS receiver. These other threads suggested that the GPS
receiver is the more accurate measurement. My question is , how is known? Has the GPS receiver speed indicator been checked against a radar gun or from some other speed measuring method?

The reason I am asking this question is because I am installing an aftermarket gauge which has the capability of measuring speed among other things. (It is also a tach and has other features). Anyway if the GPS is more accurate I could calibrate the gauge to it rather than the car speedometer .
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Old 09-06-2003, 07:02 PM   #2
perdition
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Well. A GPS measures overall speed, and is quite accurate. However, I would think that it would read very different from your spedometer in numerous conditions. Going uphill for example, or turning, or sliding. A GPS measures speed by calculating the change in global position (lattitude/longitude) divided by the change in time. A car's spedometer measures your velocity in the forward vector. If you're going 60 up a hill, your spedometer will read 60. However, a GPS will read less, depending on the incline of the hill. Does this make sense?

Long story short, it would be impossible to calibrate your new gauges to a GPS. They just work differently.

.aaron
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Old 09-06-2003, 07:08 PM   #3
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Actually, the GPS would read higher if you were going up a hill, as the north and east velocities of the horizontal movement (what your car's speedo shows you) will be added to the vertical velocity of the car moving further away from sea level, so that vector would be added to the horizontal vectors giving a total speed higher than you would actually be going!!!! *phew*
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Old 09-06-2003, 07:10 PM   #4
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Ermm, made simple GPS gives you speed in a 3-D world, using some of the most accurate position data source in the world, your speedo gives speed by an onboard sensor that is mildley accurate.
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Old 09-06-2003, 08:00 PM   #5
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hmm.ok.makes sense. Bear in mind that there is a least a 5 mph differnce between the two. When the speedometer says 70 the gps says 65. So the consensus would be to calibrate to the car speedomoter ? Split the differnce? Any opinions?
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Old 09-06-2003, 08:44 PM   #6
perdition
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So the GPS spedometers do take height into account? I wasn't aware of that. Is that part of the triangulation? I guess it would be... I just dont think I had ever seen altitude data on a GPS.

Quote:
Actually, the GPS would read higher if you were going up a hill, as the north and east velocities of the horizontal movement (what your car's speedo shows you) will be added to the vertical velocity of the car moving further away from sea level, so that vector would be added to the horizontal vectors giving a total speed higher than you would actually be going!!!! *phew*
If my new understanding is correct, the speeds would be the same, wouldn't they? That is, as long as the car is moving in a straight forward line. The car is measuring its speed along the resultant vector that the GPS will end up coming up with.

.aaron
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Old 09-07-2003, 01:06 AM   #7
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Kinda... If you are maintaining the same speed (straight line velocity) they should be. IMHO the GPS is more accurate but slower. IE, if you change your speed the GPS won't update as soon as the speedo, it takes awhile to average.
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Old 09-07-2003, 02:10 AM   #8
shrieden
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The gps gives you speed in a 3d world taking into account velocities in 3 differnet planes (x,y,z or north, east and vertical). The speedo is a 2d speed measurment, just giving you a speed across x and y or north and east. I think the best way for the speedo calibration is to find out the rolling diameter of your tires and have it calculated that way. The GPS reciever should give you instantaneous speed, it doesnt average unless you have an average speed function on the unit itself.
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Old 09-07-2003, 03:06 AM   #9
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To answer the original question minus all the propeller headed techno-babble, yes the speedo is inaccurate as compared to a GPS.

My cars speedo runs 4 mph slow as compared against both radar (stationary traffic monitoring) and my Timex/Garmin GPS based 'Speed + Distance' unit.

It was posted a long time ago though that there was some rumor that all the STi gauges were off due to a production error at Defi regarding the physical installation of the needle on to the gauge pack.

- Janq

Last edited by Janq; 09-07-2003 at 07:10 PM.
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Old 09-07-2003, 01:37 PM   #10
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Quote:
Originally posted by shrieden
The speedo is a 2d speed measurment, just giving you a speed across x and y or north and east.
Methinks you are confused. The speedo reads the linear velocity along the inclined roadway, just as the GPS receiver would do.
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