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GPS speedo accuracy

I have a GPS map thing called a Navman. It shows current speed. I have now bought a HUD thing that projects current speed onto the windscreen. A test with both running side by side showed 80kph on the HUD, and 75kph on the Navman. I thought that all these GPS gadgets were spot on. Has my faith been misplaced?

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Most GPS based devices read very similarly. In my Fg Falcons I find my Navman and GPS dashcam say I am doing 2km/hr than the speedo shows. However when I plug my dashcam into my mapping programs it shows me going the same speed as my speedo indicates so I suspect my digital speedo in my FG is very accurate. GPS devices measure speed between what is assumed to be a straight line between to fixed points so the will generally under-read as they can't take into account you may not be travelling in a straight line and have to go around bends and corners so they are usually around 2km/hr out. They also generally use an algorthym that calculates an costantly revised average speed from ongoing readings. I assume the HUD is taking it speed of a drive shaft sensor or similar rotating part and it's accuracy will depend on how well it is calibrated. If calibrated correctly it should be totally accurate but people of for they may have changed from stock wheel or tyre size or have a non stock diff etc.

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Regards Blue

Thanks Blue , the item I bought takes its information from satelites,just like my Navman. I bought it from an ebay seller in Malaysia, and when it arrived I noticed an "SCA" logo on the box. Further examination of the box showed that it was manufactured in China for Super Cheap Auto in Australia. The instruction sheet in the box was in Chinese language only. I contacted the seller, and they sent me by email an English language instruction sheet, which includes a step to adjust the unit to have shown speed coincide with that on the car speedo. I am guessing that the factory has set the speed reading fast to help minimise the risk of speeding fines, and leave it to the buyer to go closer to reality at their own risk. First time I have heard of a GPS item where you adjust the reading, but, I am no expert.

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subject to variation due to going up/down hill as satellite imagery cannot measure the triangular distance ( hypotenuse ) up/down a hill , can only calculate based on the flat ( base )distance between top and bottom of hill leading to minor discrepancy depending on how steep the grade is.

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Thanks Blue .The item I bought takes its information from satelites,just like my Navman. I bought it from an ebay seller in Malaysia, and when it arrived I noticed an "SCA" logo on the box. Further examination of the box showed that it was manufactured in China for Super Cheap Auto in Australia. The instruction sheet in the box was in Chinese language only. I contacted the seller, and they sent me by email an English language instruction sheet, which includes a step to adjust the unit to have shown speed coincide with that on the car speedo. I am guessing that the factory has set the speed reading fast to help minimise the risk of speeding fines, and leave it to the buyer to go closer to reality at their own risk. First time I have heard of a GPS item where you adjust the reading, but, I am no expert.

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Yes; that's odd.

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Regards Blue

Your vehicle is more than ten years old so therefore is allowed to be out by 10% + 4 kph under ADR. And can legally be sold as such.. NOW the thing is you get a speeding fine.. How accurate are you , how accurate is the "camera" and where does it leave you??? Its the great scam going by the government !

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