Poor Speedo Accuracy?
05 June 2003, 09:12 AM
#1
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Just got a GPS receiver, so I've been doing the obligatory comparisons between the speedo in my MY02 WRX wagon and the speed as indicated on the receiver.
Surprised to see that an indicated 100 mph (on that well known private track that we all use) is only a true 91 mph....
Slightly less surprised to see that an indicated 30 mph on the speedo is a true 27 mph. 60 mph is a true 54 etc....
So, the speedo error seems to be reasonably linear - ie. the error is approximately the same in percentage terms, no matter what the speed.
Anyone else tried comparing GPS with speedo MPH - how accurate are you?
Surprised to see that an indicated 100 mph (on that well known private track that we all use) is only a true 91 mph....
Slightly less surprised to see that an indicated 30 mph on the speedo is a true 27 mph. 60 mph is a true 54 etc....
So, the speedo error seems to be reasonably linear - ie. the error is approximately the same in percentage terms, no matter what the speed.
Anyone else tried comparing GPS with speedo MPH - how accurate are you?
05 June 2003, 09:51 AM
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Hi Hoppy - seems more accurate than mine.... I'm starting to assume that my speedo is a blatant liar 
Or, can these GPS units misread at all???
Or, can these GPS units misread at all???
05 June 2003, 09:58 AM
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My Snooper 6 Neo gives similar results on my MY94 JDM car. I think the GPS speed should be pretty accurate as far as steady speeds are concerned. It just takes a little time to work nout the new speed if you change of course. At least the speedo is on the safe side anyway, as far as points is concerned.
Les
[Edited by Leslie - 05/06/2003 09:59:50]
Les
[Edited by Leslie - 05/06/2003 09:59:50]
05 June 2003, 10:07 AM
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Very true Les - but I was just 'upset' that my privately attained 100 mph was only 90-ish.... What does that make my privately achieved 140 worth the week before? 
Cheers, HB.
Cheers, HB.
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05 June 2003, 10:40 AM
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my STiV's accuracy is really bad, according to the speedo im doing 100 mph on the M/Way in the morn & im not really passing anyone, either that or everybodys doing a ton, I'll have to change in to third gear me thinks 
05 June 2003, 10:49 AM
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I used to get that with a modded Polo with 14 inch wheels and 45 series tyres. Even mountain bikes used to pass me at an indicated 60.....
with 14 inch wheels and 45 series tyres. Even mountain bikes used to pass me at an indicated 60.....
05 June 2003, 11:22 AM
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i know speedos are not allowed to underread but overread by 10%. is this also linked to the odometer? i.e. does this mean a 100k mile car only has 91k miles ?
?
05 June 2003, 12:32 PM
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Harry,
I've just got a GPS that plugs into an Ipaq running TomTom. It too shows that the speedo is almost 10% over. I understand that it is common for most cars to be 10% over - this ensures that manaufacturers can never be accused of causing speeding offences or even accidents.
I've just got a GPS that plugs into an Ipaq running TomTom. It too shows that the speedo is almost 10% over. I understand that it is common for most cars to be 10% over - this ensures that manaufacturers can never be accused of causing speeding offences or even accidents.
05 June 2003, 12:38 PM
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Hi Fazack - that's exactly the set up I've got. Have you visited the Tomtom site to get the latest software revisions?
05 June 2003, 02:43 PM
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Fazack - how many satellites are normally visible on yours? I usually get 6 or 7 - but have detected up to nine on the A1, M1 and M62.
06 June 2003, 10:51 AM
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Yes I know how you feel about what speed you really achieved HB, and I am wondering exactly the same about how fast I really got up to on an old runway. have to try it again with the GPS I suppose.
Les
Les
06 June 2003, 11:03 AM
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I believe that it is more likely that the gps will be wrong, considering the small delay in sending and recieving signals from satelite (or however they do it). I would assume that if they can be a couple of meters out on positioning surely they could be out with calculating speed?
06 June 2003, 02:12 PM
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Could be Ian - that's what I'm trying to establish. Otherwise, what's the point of having a WRX that only does 60 in seven secs, or 100 in 20...
06 June 2003, 02:44 PM
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Ian,
The time delay is pretty negligable(radio waves travel at the speed of light) and modern GPS units lock onto 4 or more satellites (3 for triangulation plus 1 or more for accuracy and backup). The one thing GPS is rubbish at is height because the time delay difference between 100ft and say 25ft is so small it's very hard to be accurate. Lat and long is easier because of triangulation.
At least that's how I understand it to work.
The time delay is pretty negligable(radio waves travel at the speed of light) and modern GPS units lock onto 4 or more satellites (3 for triangulation plus 1 or more for accuracy and backup). The one thing GPS is rubbish at is height because the time delay difference between 100ft and say 25ft is so small it's very hard to be accurate. Lat and long is easier because of triangulation.
At least that's how I understand it to work.
06 June 2003, 04:01 PM
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The only reason I say that is because every car I have been in with GPS, freelander, beamer and renault. The speedo has always clocked the car at about 10% difference form the GPS.
09 June 2003, 09:16 PM
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It would be nice to get some confirmation on this, I am sure I've read that most cars are 10% over; imagine if a car speedo was under - the manaufacturers would have their a??e sued off them by everyone with a speeding ticket.
09 June 2003, 09:22 PM
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Ipaq5450 + Navman3000 + Tom Tom II .
MY01 WRX always 12% overreading ! .... Which is not too bad ... however now I know it over-reads I am tempted to go faster
Scott
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MY01 WRX always 12% overreading ! .... Which is not too bad ... however now I know it over-reads I am tempted to go faster
Scott
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10 June 2003, 10:40 AM
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Can't you check how accurate the speedo reading is by using the indicated engine revs and the gear ratios to calculate the speed ?
I think it's something like 3000 rpm in top gear is 76mph and 4000 rpm is 100mph (on a classic scoob).
I think it's something like 3000 rpm in top gear is 76mph and 4000 rpm is 100mph (on a classic scoob).