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      08-12-2012, 11:06 AM   #1
John320d
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GPS check on speedo accuracy.

Hi guys I would like to do the above but not got a clue how to. I take it is carried out by using the sat nav. Is it possible to do it with the Business nav I have installed in my 58 reg 3 series? Many thanks in advance for your replies.
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      08-13-2012, 02:47 AM   #2
doughboy
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Your speedo will read about 3 to 5 mph too high, they all do, they are made to do that as its the law.

Under Eu law, all speedos must over-read between +5% to -0%, so most are about +2.5% (of full scale) or about 3 to 5 mph.

If you want to try it, best use a independant sat nav, or an app on your phone.

Do it in a straight line at a steady speed for best accuracy.

Remember the biggest effect on it will be your rear tyre tread depth, new tyres make the speedo more accurate, as they wear the speedo reads higher for the same actual speed.
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      08-13-2012, 07:48 AM   #3
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Thankyou doughboy for replying. Doh, some pillock did not ask the right question.The query I really had was about checking the odometer accuracy at the moment, irrespective of tyre wear etc.
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      08-13-2012, 09:00 AM   #4
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Garage List
speak to you highways dpt or taxi licensing dpt for the nearest measured mile,
might be on their web sites
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      08-13-2012, 09:06 AM   #5
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It will be about 3 to 5 mph out (reading high), unless your tyres are very low, then it might be 6 or 7 mph out.

GPS is still only accurate to +/- 1mph, so there's error there too.

If you have built in nav, there is a way to get GPS speed on the ODB screen with the secret menus, search on here.
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      08-13-2012, 09:07 AM   #6
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odometers in general are much much much more accurate. They dont have to satisfy the whole 5% rule and as such are as close to dead on as the laws of physics will allow (e.g. differing tyre size and wear etc).
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      08-13-2012, 09:15 AM   #7
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Also, people seem to think a GPS speed is gospel but it isn't - it depends not only on the resolution of the GPS receiver but also how frequently it measures its position.
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      08-13-2012, 09:26 AM   #8
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According to the GPS on phone, My Escort 95001x, and Garmin my spedo is off 4% which I plan to have fixed through coding.

It still tracks miles properly which annoys me that the speedo is the only part off.
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