Satellite Navigation Speed Accuracy
13 September 2007, 08:56 AM
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Satellite Navigation Speed Accuracy
Hello
How accurate is the speed reading on a satellite navigation unit (I have a Nuvi)?
I'm asking as I'm suprised at how far out my speedo is. It has been converted from KPH to MPH years ago (imported Supra) but when it reads 90 I'm doing about 80 according to the satnav.
Steve
How accurate is the speed reading on a satellite navigation unit (I have a Nuvi)?
I'm asking as I'm suprised at how far out my speedo is. It has been converted from KPH to MPH years ago (imported Supra) but when it reads 90 I'm doing about 80 according to the satnav.
Steve
13 September 2007, 09:00 AM
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Hello
How accurate is the speed reading on a satellite navigation unit (I have a Nuvi)?
I'm asking as I'm suprised at how far out my speedo is. It has been converted from KPH to MPH years ago (imported Supra) but when it reads 90 I'm doing about 80 according to the satnav.
Steve
How accurate is the speed reading on a satellite navigation unit (I have a Nuvi)?
I'm asking as I'm suprised at how far out my speedo is. It has been converted from KPH to MPH years ago (imported Supra) but when it reads 90 I'm doing about 80 according to the satnav.
Steve
13 September 2007, 09:08 AM
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I'd agree with the above assertion (GPS more accurate) BUT I'd give it the proviso that you should be travelling in a straight line at a constant speed. If you're cornering then it will be calculating distance via 2 gps locations which, and I could be wrong, it assumes are in a straight line for the purposes of the speed calculation. So you've actually travelled further than the GPS system thinks you have ....
That may all be bill hooks though but it's the way I loo at seeing how accurate my speedo is.
Dave
PS: the Passat says 80 on the speedo (private road ossifer) and the GPS says 77/78. I need to attach it to my mates Moto Morini though as the speedo on that is way out ...... but I'd like to know by how much ...
That may all be bill hooks though but it's the way I loo at seeing how accurate my speedo is.
Dave
PS: the Passat says 80 on the speedo (private road ossifer) and the GPS says 77/78. I need to attach it to my mates Moto Morini though as the speedo on that is way out ...... but I'd like to know by how much ...
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13 September 2007, 07:14 PM
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I've got 3 gps devices in my car (the tomtom, my radio, and my phone) and all three read the same, generally about 5mph slower than the speedo...
14 September 2007, 08:31 AM
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I'd agree - I think there is a 2 second delay on GPS readings which is why, as stated, you have to hold the speed at a set amount for a decent period of time. 80 is a good indicator - as the higher the speed the greater the potential fluctuation. So stick at 80 on the speedo, and then read the GPS figure and that will tell you all you need to know.
When I had my first Tomtom and b2, both were exactly the same reading of speed - in my Porsche 993, at 80 mph the Porsche read 84. I had a Rover 214 as well as a daily driver, and that read 81mph. Always amazed me that a £40k car was less accurate than a £900 car. But then Rover did get something right I guess.
When I had my first Tomtom and b2, both were exactly the same reading of speed - in my Porsche 993, at 80 mph the Porsche read 84. I had a Rover 214 as well as a daily driver, and that read 81mph. Always amazed me that a £40k car was less accurate than a £900 car. But then Rover did get something right I guess.
14 September 2007, 08:36 AM
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He doesn't understand?
There was a discussion about this some time back, do a search for posts by "hedgehog" on GPS. It seems there are 2 types, 1 of which can give erroneous readings but that type isn't generally used in cars / handheld devices. As previously covered, at a steady speed in a straight line it is very accurate, if you look in you TomTom manual etc it will give you the stated tolerance and at up to 90mph it's less that 1mph variance IIRC.
There was a discussion about this some time back, do a search for posts by "hedgehog" on GPS. It seems there are 2 types, 1 of which can give erroneous readings but that type isn't generally used in cars / handheld devices. As previously covered, at a steady speed in a straight line it is very accurate, if you look in you TomTom manual etc it will give you the stated tolerance and at up to 90mph it's less that 1mph variance IIRC.
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