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How accurate is the speed shown?
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keithc
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PostPosted: Sat Nov 20, 2004 4:05 pm    Post subject: How accurate is the speed shown? Reply with quote

I see a difference of approximately 8% when driving. My speedometer showing 100 km/h and ttn3.07 showing 92 km/h. Is the ttn display accurate? Is there an allowed tolerance of +/- n%?
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Keith
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lbendlin
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PostPosted: Sat Nov 20, 2004 4:07 pm    Post subject: Reply with quote

TTN is accurate towithin 0.5%. But it's not continuous - data is only supplied every second. If you break hastily this will not reflect correctly on GPS
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nighthawk
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PostPosted: Sat Nov 20, 2004 5:50 pm    Post subject: Reply with quote

Tom tom is probably more accurate than your speedo

I know my car is doing exactly 70mph when the speedo shows 74mph
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DavidW
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PostPosted: Sat Nov 20, 2004 6:21 pm    Post subject: Reply with quote

The UK (and I believe EU) specification for car speedometers is that they must not read under your true speed, but can read over your true speed by up to 10%. As a result, many speedometers over-read by a fair few percent.

Your GPS speed is likely to be rather more accurate than your car speedo, though it is possible for the GPS speed to be a little off in either direction.



David
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Robin2
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PostPosted: Sat Nov 20, 2004 7:01 pm    Post subject: Reply with quote

If you are travelling at a steady speed on the level TomTom speed is likely to be very accurate (better than +/- 1 mph, but it will underestimate your speed if you are going up or down hill.
I set my cruise control to TomTom 70 mph on the motorway, the speedo is reading 74 on my Golf. In my experience, Fords seem to have the fastest speedos, often as much as 10% high. Probably to keep the road warriors happy!
Robin
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uffe73
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PostPosted: Sat Nov 20, 2004 10:26 pm    Post subject: Reply with quote

I've seen the same behaviour in my car (a Volvo S60) even though I'm running Navman's SmartST instead of TomTom. The car speedometer shows up to 10 km/h more than SmartST. Can anyone tell me what the best theoretical speed accuracy is in a GPS based navigation system when you're driving at constant speed and constant height above sea level on a straight road?

The resolution in SmartST is only 3-4 km /h which might be due to a rounding error in the SW (I've reported this to Navman). What is the resolution of the speed indication in TomTom?

/Ulf
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andrewh
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PostPosted: Thu Nov 25, 2004 5:01 pm    Post subject: Reply with quote

The resolution in Tomtom for Palm is 1 km/h

Interestingly I see my Audi A6 speedo is very accurate up to 100 km/h and then teh scale changes and it starts increasingly to under-read. WhenI am at a true (TT) 140 km/h the speedo shows almost 150.

I never liked the non-linear (or should I say non-equal-angular) speedo before I had a Tomtom, now I like it less!

Andrew
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jpkeates
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PostPosted: Fri Nov 26, 2004 2:53 pm    Post subject: Reply with quote

A straw poll shows a consistent 10% overstatement on speedometer speed against true (GPS) speed.

This is even true of my motorbike.
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lbendlin
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PostPosted: Fri Nov 26, 2004 4:00 pm    Post subject: Reply with quote

uffe73 wrote:
Can anyone tell me what the best theoretical speed accuracy is in a GPS based navigation system when you're driving at constant speed and constant height above sea level on a straight road?

The NMEA stream reports speed in knots with three digits after the decimal point. It is generally ok to assume that the accuracy is better than 0.1 knots - make that better than 0.05 knots - but only under the assumed condition.
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weelogic
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PostPosted: Tue Nov 30, 2004 12:56 pm    Post subject: Reply with quote

I am sure I read somewhere that speedos in the UK are required by law to read at least 2.5mph over the true speed. Certainly that seems a common difference between my Saab speedo and TT.
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daveyft
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PostPosted: Tue Nov 30, 2004 1:58 pm    Post subject: Reply with quote

I've used TomTom in several cars in the UK and they all seem to overread by about 8%ish. Dont know if this is something to do with the law here but I also used it in a Cadillac DeVille in the US, it had a digital speedo and that was spot on with TomTom.
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Skippy
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PostPosted: Tue Nov 30, 2004 2:14 pm    Post subject: Reply with quote

The law is that a speedometer must read 0% to 10% fast and in my experience, the accuracy can be anywhere in this range.

A GPS receiver will report your speed to within .1 MPH when traveling on flat ground in a straight line at a constant speed. Note that the speed is reported by the receiver, not calculated by the application.
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MarkHewitt
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PostPosted: Tue Nov 30, 2004 2:55 pm    Post subject: Reply with quote

I've noticed the phenomenon that it doesn't give you an accurate ground speed when going up or down hill. Since GPS receivers should know your altitude is there a reason they don't compensate for this and show your correct speed?

I think you are right about Ford speedos overreading, I went for a trip at the weekend with my TomTom and found that I had to travel at at least 65mph as incidated by my speedo before my GPS said I was travelling at the 60mph speed limit.

This is very useful information, especially in these days of speed cameras etc. Being able to that extra 5mph legally can make quite a difference to your progress over long distances.
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MrT
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PostPosted: Tue Nov 30, 2004 4:02 pm    Post subject: Reply with quote

MarkHewitt wrote:
I've noticed the phenomenon that it doesn't give you an accurate ground speed when going up or down hill. Since GPS receivers should know your altitude is there a reason they don't compensate for this and show your correct speed?


On a 1 in 10 hill the error due to the gradient would be 0.5% so it makes little difference.
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MarkHewitt
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PostPosted: Thu Dec 02, 2004 4:48 pm    Post subject: Reply with quote

Really? Well on my recent trip it seemed to make quite a bit of difference, about 3 - 4mph difference in fact, and just going up and down gently rolling hills, not steep ones.
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