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help with vehicle speed

 
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scraggy
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Joined: May 09, 2004
Posts: 3

PostPosted: Fri Oct 08, 2004 7:53 pm    Post subject: help with vehicle speed Reply with quote

Hi
I have just fitted a ipaq 2210 with TT3 bluetooth to my truck, but I have noticed that the speed given in TT3 is a long way from what the truck is actually doing.
Is there anyway of getting a more acurate reading, as I would like to use it as a guide to get a acurate speed on some other machines I have.
Regards
turfman
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tOpCaT
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Location: Bucks

PostPosted: Fri Oct 08, 2004 9:23 pm    Post subject: Reply with quote

Are you sure about that? It might be the truck speedo.

The speedo in my car is WAY OFF. It consistently reports about ten percent higher than the actual speed. I've gone past the speed camera on the top end of the A43 at 70mph (reported as 77mph on the car speedo) and not been flashed...

Thinking about it, this explains why it often takes me longer to do a journey than I think it should - my average speed is lower than I think it is!
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lexuscol
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Joined: Jun 25, 2004
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PostPosted: Fri Oct 08, 2004 10:35 pm    Post subject: Reply with quote

the gps speed is more accurate than your speedo, providing you have a good satellite fix.
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LeeB
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Joined: Oct 03, 2004
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PostPosted: Sat Oct 09, 2004 11:03 am    Post subject: Reply with quote

tOpCaT wrote:
Are you sure about that? It might be the truck speedo.

The speedo in my car is WAY OFF. It consistently reports about ten percent higher than the actual speed. I've gone past the speed camera on the top end of the A43 at 70mph (reported as 77mph on the car speedo) and not been flashed...

Thinking about it, this explains why it often takes me longer to do a journey than I think it should - my average speed is lower than I think it is!


I agree with lexuscol, the GPS is far more accurate than your vehicle's speedometer, whether it be a car or commercial vehicle.

Construction and use regulations apply to cars and prescribe that the speedo must indicate to a maximum error of 10% of the vehicle's speed at all speeds above 10mph.

Tachograph rules applying to commercial vehicles are the subject of EU regulation and are stricter - the accuracy is to a maximum error of 6km/h across the entire range.

So, whilst a truck's tachograph can be more accurate than a car's speedometer, they both allow for errors whereas the GPS method has a much higher degree of accuracy and the faster you are travelling, the more accurate is its reporting.

This is anorak stuff, I know, but - hey! Wink
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amcluesent
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PostPosted: Sun Oct 10, 2004 5:45 pm    Post subject: Reply with quote

The accuracy of the speed display in TomTom was discussed on that forum a few weeks ago, the general view that on a straight, flat road the GPS was very accurate.

I know someone who 'calibrated' his car speedo, by using a china-pencil to mark the true 30, 40, 70mph on the bezel. If you were a gambler, you could add on the ACPO prosecution tolerance of 10% + 2mph before marking the speedo... 8O
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LeeB
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PostPosted: Sun Oct 10, 2004 6:13 pm    Post subject: Reply with quote

amcluesent wrote:
The accuracy of the speed display in TomTom was discussed on that forum a few weeks ago, the general view that on a straight, flat road the GPS was very accurate.

I know someone who 'calibrated' his car speedo, by using a china-pencil to mark the true 30, 40, 70mph on the bezel. If you were a gambler, you could add on the ACPO prosecution tolerance of 10% + 2mph before marking the speedo... 8O


Confused I'd be wary of that, you could have a car speedo indicating 70mph and be travelling at 77mph If you add the 2mph you'd possibly be travelling at 79.2mph if the inaccuracy is consistent.

I know you hear stories of "no prosecutions unless you're above 80mph on the motorway" but I've already got three points on my licence and wouldn't want six!

Sad Without wanting to sound pompous, if your speedo was under-indicating then showing 30mph could be 33mph actual and all the TV ads show what that means. I think I'd better not say any more as I'll sound like some sort of campaigner! Smile

Like you said, "If you're a gambler ..."
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dave2318
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PostPosted: Mon Oct 11, 2004 2:02 pm    Post subject: Reply with quote

I have been doing tests on this, as my TomTom is also about 10% slower than my speedo records. I also have to record accurate mileage for my job (from one place to another). The general results are quite interesting. I just had two new front tyres fitted and the difference it has made is amazing! My TomTom speed at 30 miles an hour used to be 25-26 now it is 27-28 (obviously depending on cornering and going up/down hill. The distance from one place to another has also changed! It seems that even the depth of tread and the pressure in your tyres makes a car speedometer change.
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tOpCaT
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PostPosted: Mon Oct 11, 2004 4:30 pm    Post subject: Reply with quote

Even though we all know that pressure/tread differences must make a difference to the effective tyre circumference and hence the reported speed and distance a car speedometer shows, it's still amazing to see what a difference is actually made, isn't it?

If you get reimbursed for mileage I'd say go with whichever is highest! Wink
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Darren
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PostPosted: Mon Oct 11, 2004 6:52 pm    Post subject: Reply with quote

On a level road at constant spped with good satellite coverage you can expect the GPS reported speed to be far more accurate than your vehicle speedometer which has a built-in error of up to 10%.

This is not the case if you are climbing or descending, accelerating or braking or have poor satellite coverage all of which can have an effect on accuracy of the GPS reported speed.
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turfman
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PostPosted: Mon Oct 11, 2004 8:25 pm    Post subject: Reply with quote

Hi everyone
Thanks for the replies, I have tried it yesterday in one of the tractors I know its exact speed and it was spot on accurate. which makes the truck speedo out by a heck of a lot.
Regards
turfman
PS I know I have a different alias this time, I thought I had regestered here and used my usuall nick and password only to discover I had not regestered but had used the same details as another user and when I posted the first message I was logged on from thier account.
so appologies.
Embarassed
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xda
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Location: Park Gate

PostPosted: Tue Oct 12, 2004 6:15 am    Post subject: Reply with quote

LeeB wrote:
Confused I'd be wary of that, you could have a car speedo indicating 70mph and be travelling at 77mph If you add the 2mph you'd possibly be travelling at 79.2mph if the inaccuracy is consistent."


You will find that vehicle manufacturers by law have to callibrate speedometers such that it is illegal for the true speed of the vehicle to exceed the indicated speed, with the vehicle on manufactures recommended wheels and tyre sizes.

Statutory Instrument 2001 No. 25
The Motor Vehicles (Approval) Regulations 2001

1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at uniform intervals not exceeding 20 mph at all speeds up to the maximum speed of the vehicle and capable of being read by the driver at all times of the day or night.

2. For all true speeds up to the design speed of the vehicle, the true speed shall not exceed the indicated speed.

3. For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed -

V/10 + 6.25 mph

where V = the true speed of the vehicle in mph.
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Graham.
TT Go720, App:9.510(1234792.1) OS:842337
GPS: V1.20, Boot: 5.5279, Home: V2.9.5.3093
Map: Europe V910.4892
Map: Europe_Truck V870.3421, Kingston 8GB SD
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HandyMac
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PostPosted: Tue Oct 12, 2004 7:41 am    Post subject: Reply with quote

I was checking this out last night. Very unscientific test over a short distance of a couple of miles.

My speedo suggested a road speed of about 48mph. TomTom3 reported about 42mph.

It's a Freelander which has some similarities to a farm tractor on steroids.

Andrew
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xda
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PostPosted: Wed Oct 13, 2004 6:57 am    Post subject: Reply with quote

HandyMac wrote:
My speedo suggested a road speed of about 48mph. TomTom3 reported about 42mph.

At an indicated speed of 75mph CoPilot is showing 70.9mph and at 30mph CoPilot shows 28.4mph.
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Graham.
TT Go720, App:9.510(1234792.1) OS:842337
GPS: V1.20, Boot: 5.5279, Home: V2.9.5.3093
Map: Europe V910.4892
Map: Europe_Truck V870.3421, Kingston 8GB SD
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Kernel
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PostPosted: Wed Oct 13, 2004 11:41 pm    Post subject: Reply with quote

I would be prepared to go with the GPS indicated speed - here in New Zealand a driver was recently able to produce a GPS speed profile (that he had shown to the police officer at the time of the offence) indicating that he had been travelling within the tolerance allowed (100+10 km/h).

The court accepted that the GPS speed was more accurate than the doppler radar system used in a moving police car and dismissed the case, a much better option than the $400 odd fine (and demerit points) he would have otherwise been up for.

I suspect that many more people in this country are now driving with GPS units turned on!

Mike.
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davenash
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Joined: Aug 12, 2004
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PostPosted: Thu Oct 14, 2004 12:22 am    Post subject: Reply with quote

tOpCaT wrote:
Are you sure about that? It might be the truck speedo.

The speedo in my car is WAY OFF. It consistently reports about ten percent higher than the actual speed. I've gone past the speed camera on the top end of the A43 at 70mph (reported as 77mph on the car speedo) and not been flashed...

Thinking about it, this explains why it often takes me longer to do a journey than I think it should - my average speed is lower than I think it is!


Ive been through that cam at 90 - its not always on

Dave
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