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MadJax
Member Since: 30 May 2011
Location: Adelaide
Posts: 120
  
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| Speedo Speed vs GPS Speed |
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Is it just me or does my speedo show a slower speed than my GPS device 
I do need to do some more testing but I believe a 10% difference is shown, the speedo being slower.
Not only that but I cannot find a Sat Nav function that shows speed. Which is interesting.
What are your views on this
Thanks  Discovery 4 2.7 SE
Defender 130 TD5 - Daily Drive
2.7t Caravan
Java TD5 HSE - Gone but not forgotten.
Java V6 Freelander - Had fun in it.
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 15th Jun 2011 12:15 pm |
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Voug
Member Since: 03 Dec 2010
Location: Somewhere between East and West
Posts: 195
 
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Checked yesterday my blackvue recordings ... 120 km/h at the cruise control / speedo gives 117 km/h via GPS. Thats okay.
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| 15th Jun 2011 12:33 pm |
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DG
Site Moderator
Member Since: 12 Dec 2005
Location: The Gaff
Posts: 50979
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Your speedometer should never read less than your true speed .......but it is allowed to display more than your true speed by a certain percentage (well, under EU regs, not sure about AUS) 21 year LR veteran > D2 GS 2003 > D3 S 2006 > D3 HSE 2009 > D4 HSE 2013 > D4 HSE 2015 > D5 HSE 2018 > DS HSE R-Dynamic P300e 2021
Last edited by DG on 16th Jun 2011 12:21 am. Edited 1 time in total
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| 15th Jun 2011 12:45 pm |
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RichardB
Member Since: 03 May 2010
Location: Hants
Posts: 485
 
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I've been using GPS units in cars for 20 years odd. Before all 24 satellites were in place and well before Selective Availability was switched off officially by Bill Clinton and tomtom was running on a psion 3.
Never yet had a car where the GPS registers lower than the speedo. As DG says, speedos must not under report speed for obvious reason. So they are pretty universally calibrated to overreport. Usually around 5% to a max of 10%. My D4 seems to pretty reliably do 70 GPS (true) when the clock gives 70 and 50 is 53/54. mph that is. Useful to know for the UKs average speed check cameras.
Clearly, the speedo does not take a feed from the in car GPS! I also suspect it's why built in units don't show speed on the Nav display from the GPS.
May be obvious but GPS units derive speed indirectly from 2 position readings, usually about 1 second apart or so. They may also overlay this with a moving average to provide damping. Consequently GPS speeds lag true speed,
so make comparisons when running at a steady speed!
As a result, if your speedo is under reading consider getting it checked especially if under warranty.
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| 15th Jun 2011 8:38 pm |
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kenny
Member Since: 17 Jan 2010
Location: South West Aus
Posts: 79
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From memory the speedo is set to +3% +3kph here in Aust on the CCF. If you have the touch screen and set it to diagnostic mode there is a screen in there that will show the GPS speed and the MOST Buss speed. Those two speeds are almost identical to each other, it's only the dashboard display that is higher.
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| 15th Jun 2011 10:36 pm |
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MadJax
Member Since: 30 May 2011
Location: Adelaide
Posts: 120
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I should have looked at the diagnostic screen earlier.
Will try it over the weekend Discovery 4 2.7 SE
Defender 130 TD5 - Daily Drive
2.7t Caravan
Java TD5 HSE - Gone but not forgotten.
Java V6 Freelander - Had fun in it.
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| 15th Jun 2011 11:36 pm |
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hughes_jh
Member Since: 09 Jan 2011
Location: Warwickshire
Posts: 294
 
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Speedo needs to show a tad higher than actual to allow for any uncontrollable variation, even things like tyre wear will affect the speedo (the theory being that your car goes faster on new tyres due to the larger diameter)
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| 16th Jun 2011 8:04 am |
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