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Topic: Speedometer accuracy (Read 952 times) |
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Neil W
Full Member
Posts: 148
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Speedometer accuracy
« on: Mar 6th, 2005, 5:49pm » |
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On the Scorpio simple.lists site there was a thread about speedometer accuracy, or rather a built in inaccuracy on all modern cars. So I decided to check mine using differential GPS, driving at constant speeds. GPS is, of course, very accurate over more than one refresh cycle.
The results were interesting:
GPS speed Speedometer speed
30 mph 33mph
60 mph 65mph
70mph 76mph
I tried them twice on a dual carriageway, once in each direction (to cancel out wind effect  )
Has anyone else found this?
Neil.
Junior member at last 
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1998 Scorpio Ultima Cosworth facelift saloon, reg S896VOG. Sold Feb 2010. Now own Mercedes-Benz E320 CDi.
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Mottsy
Junior Member
Posts: 78
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Re: Speedometer accuracy
« Reply #1 on: Mar 6th, 2005, 5:54pm » |
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I noticed that when travelling on motorways at an indicated 60mph, lorries were always slowly cating up with me. This lead me to believe that I was in fact traveling at around 55mph.
A check along a measured mile produced results consistant with those you have found.
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Geoff_W
Senior Member
98 Ulti2.5tdi Auto
Posts: 1340
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Re: Speedometer accuracy
« Reply #2 on: Mar 6th, 2005, 7:01pm » |
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Yes Neil,
Those numbers back up my GPS experience.
Geoff
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The winding road goes on and on... or at least it does in Northumberland
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Aardvark
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Click here to make a clicking noise
 
Posts: 48
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Re: Speedometer accuracy
« Reply #3 on: Mar 6th, 2005, 8:50pm » |
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A speedo over-reading of +4% / +8% is typical for most cars nowadays, consistent with your results.
An "actual speed" code can be accessed from the ECU via most SEAT/Skoda/VW/Audi climate control systems. This usually bears out an over-reading of +8% .
However my Alhambra speedo only reads +4%, thus when driving that I usually gain on most other cars through 40mph speed restricted roadworks etc but not in the Leon R which over-reads 8%!
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'95 Scorpio Ultima 2.0 16V - Tourmallard Green
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proberjohn
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No Scorpio any more -(
Posts: 192
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Re: Speedometer accuracy
« Reply #4 on: Mar 6th, 2005, 8:51pm » |
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Same for me - a full 10% out when compared with the GPS. No difference with higher tyre pressures either. Real pain when trying to be legal but hurrying along.
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geraghtyg
Full Member
2001 XG25
Posts: 215
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Re: Speedometer accuracy
« Reply #5 on: Mar 6th, 2005, 9:43pm » |
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I checked this with GPS and got similar results. Max speed on motorways over here is 120KM/H, which is about 74.5MPH. However, I can drive at an indicated speed of about 81MPH and be actually driving @ 74.5MPH. Interestingly, the speedo in the advanced diagnostics screen (where you get litres of fuel left, etc) gives a fairly accurate estimation of the current speed as 81 MPH also shows up as 120KM/H on this.
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Graham G, Dublin, Ireland
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Thunderchild
Full Member
My other car's an Aprilia
Posts: 199
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Re: Speedometer accuracy
« Reply #6 on: Apr 1st, 2005, 11:17am » |
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Same for me with the GPS. Mine is out by a fraction under 10%. Generally now when cruising on the motorway I rely on the GPS and cruise control to keep me on the limit. Most cars still fly past me though but hey what do I care LPG and the equivalent of about 45 to the gallon from my cossie is good for me!!! That and loads of grunt when you want it.... lovely.
Thunderchild
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ChrisB
Junior Member
Without a scorp !
Posts: 58
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Re: Speedometer accuracy
« Reply #7 on: Apr 2nd, 2005, 10:44pm » |
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You get the same thing if you hook a ODB lead up and measure the VSS sensor which gives true speed. It appears to be adjusted at the speedo head to read slightly high.
At least it means your unlikely to get a photo unless your doing something silly
Chris B
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Dave
Senior Member
Tourmallard 24v (ex-owner)
Posts: 1984
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Re: Speedometer accuracy
« Reply #8 on: Apr 2nd, 2005, 11:16pm » |
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You can also press and hold the didgery stick (the trip distance resetter) and cycle through the functions, one of them gives you your actual speed in KM/hour! It's like having a digital dash! This also gives the more likely accurate figure, about 10% less than the analogue type speedo.
D
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Life after the Scorpio ain't easy..
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Danny_R
Full Member
97 Ford Crown Vic 4.6L V8 ,88 Mercury GM 5.0L V8
Posts: 381
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Re: Speedometer accuracy
« Reply #9 on: Apr 4th, 2005, 1:39pm » |
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I've often wondered why the speedo's are not always dead on accurate. In America, all law enforcement vehicles like my police cruiser have certified calibration. I've tested it alongside a GPS unit and its bang on accurate at any speed. I wonder why they cant do the same in all vehicles?
Danny_R
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nic
Guest
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M8,
I had a Mk1 Granada 3.0s ex police, with a huge calibrated speedo on the dash.
I often wonder if it was right, if so lucky i never got caugh, although i did get wrong for not driving in a straight line, it went sideways if memory serves me right.
nic
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Scorpioen84
Full Member
Saloon 2.0 16v TC 140bhp -95
Posts: 404
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Re: Speedometer accuracy
« Reply #11 on: Apr 6th, 2005, 2:55pm » |
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ive found that the DIGITAL speedometre (when holding the reset button og tripcounter for 10sek) is werry acurate, maby 1 or 2 km/t to fast...
100km/t on needle gives 92km/t on Digital...
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