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deeranwith Lifetime Member
Joined: Mar 07, 2006 Posts: 32
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Posted: Thu Jan 06, 2011 2:42 pm Post subject: Speed, which is more accurate? |
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For a time now I have noticed that both my Sat Navs speed readings are slower than the speedometers in the cars. The Sat Navs both agree with each other and read approx 4-5 mph slower at say 80 mph than the speedometers (one is a digital readout model on a Toyota Yaris).
Which is the more accurate? Is there a significant lapse or error built into the Satellite system? Or is it the poor accuracy of production speedos? |
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pcaouolte Frequent Visitor
Joined: Dec 27, 2006 Posts: 998 Location: South Lincs, UK.
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Posted: Thu Jan 06, 2011 2:57 pm Post subject: |
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This has been covered a few times (at great length).
The speed displayed on the satnav is more accurate than the speed displayed on the speedometer if they differ. In the UK a speedometer can be callibrated by the manufacturer to show the speed within 0% to +10% therefore if you are travelling at 70mph your satnav will show 70mph but your car speedometer can show anything between 70mph and 77mph.
You shouldn't be travelling at 80mph. _________________ Paul |
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deeranwith Lifetime Member
Joined: Mar 07, 2006 Posts: 32
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Posted: Thu Jan 06, 2011 3:09 pm Post subject: |
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I know, it was for test purposes.
Thanks. |
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Meter_Man Lifetime Member
Joined: Mar 10, 2008 Posts: 43
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Posted: Fri Jan 07, 2011 10:21 am Post subject: |
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deeranwith wrote: | I know, it was for test purposes.
Thanks. |
_________________ Navigating by Sony SatNav (built in)
and iPhone SE with Google Maps or Navmii |
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MaFt Pocket GPS Staff
Joined: Aug 31, 2005 Posts: 15145 Location: Bradford, West Yorkshire
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Posted: Fri Jan 07, 2011 12:12 pm Post subject: |
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Maybe he was on a test track? |
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deeranwith Lifetime Member
Joined: Mar 07, 2006 Posts: 32
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Posted: Fri Jan 07, 2011 12:45 pm Post subject: |
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Good point. |
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pcaouolte Frequent Visitor
Joined: Dec 27, 2006 Posts: 998 Location: South Lincs, UK.
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Posted: Fri Jan 07, 2011 7:04 pm Post subject: |
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Re-reading th OP he wasnt doing 80 anyway he was actually doing 4-5mph less than that as indicated by the satnav. _________________ Paul |
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Skippy Pocket GPS Verifier
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
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Posted: Sat Jan 08, 2011 3:16 am Post subject: Re: Speed, which is more accurate? |
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deeranwith wrote: | For a time now I have noticed that both my Sat Navs speed readings are slower than the speedometers in the cars. The Sat Navs both agree with each other and read approx 4-5 mph slower at say 80 mph |
I didn't know the Yaris could go that fast! I'm sure this was while you were on holiday in France or Germany where the speed limit is higher than 70 MPH. ;)
The answer is that on flat ground, at a constant speed and direction, with a good satellite fix your GPS is typically accurate to within about .1 MPH.
This was discussed at length a few times, check out this link. _________________ Gone fishing! |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Sat Jan 08, 2011 4:54 am Post subject: |
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If you do the maths even being on a fairly steep incline or going round bends will make very little difference to the SatNav's calculated speed.
However the calculations are only done once a second IIRC, so maintaining a constant speed is important (or to put it another way, assume that the speed it displays will lag behind while you are changing speed) |
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pcaouolte Frequent Visitor
Joined: Dec 27, 2006 Posts: 998 Location: South Lincs, UK.
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Posted: Sat Jan 08, 2011 10:16 am Post subject: |
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Andy_P wrote: | ...assume that the speed it displays will lag behind while you are changing speed) |
I don't think he'll get much lag in Yaris? The rate of acceleration, especially when already doing around 80mph is not likely to be enough to notice? _________________ Paul |
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cwog Regular Visitor
Joined: Sep 08, 2009 Posts: 102 Location: UK
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Posted: Sat Jan 08, 2011 8:05 pm Post subject: |
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I have noticed the indicated speed difference when the tyres on the car were close to the legal limit, after fitting new tyres the Satnav indicated speed and the car speedo indicated speed were very close. |
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pcaouolte Frequent Visitor
Joined: Dec 27, 2006 Posts: 998 Location: South Lincs, UK.
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Posted: Sun Jan 09, 2011 9:42 am Post subject: |
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Tyre wear must make a difference to indicated mph if the measurement is taken from the number of revolutions that a wheel makes to cover one mile.
On my car a new tyre is 631.9mm diameter and makes 2546.8 revolutions to cover one mile. A tyre worn to the legal limit would be 619.1mm diameter and would need to make 2599.5 revolutions to cover one mile. A difference of 2% between the number of revolutions. This should, I think, give a difference of 2% between the indicated mph on my vehicle with new tyres and with worn tyres. _________________ Paul |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Sun Jan 09, 2011 10:02 am Post subject: |
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pcaouolte wrote: | On my car a new tyre is 631.9mm diameter and makes 2546.8 revolutions to cover one mile. A tyre worn to the legal limit would be 619.1mm diameter and would need to make 2599.5 revolutions to cover one mile. | Have you taken into consideration the rolling radius of the tyre which will be smaller than the static radius of the tyre due to the fact that it is a bit flat at the bottom, and the fact that the rolling radius of the tyre will increase slighty with speed due to centripedal force? |
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cwog Regular Visitor
Joined: Sep 08, 2009 Posts: 102 Location: UK
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Posted: Sun Jan 09, 2011 11:33 am Post subject: |
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and also the difference in tyre pressure my change effective rolling radius all go to indicate that that the car speedo speed is dependent on many variables hence the latitude in the claimed accuracy of the speedo. |
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pcaouolte Frequent Visitor
Joined: Dec 27, 2006 Posts: 998 Location: South Lincs, UK.
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Posted: Sun Jan 09, 2011 8:06 pm Post subject: |
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No! I can't be bothered with all that maths
Let's assume that the effects of the rolling radius and centrifugal force and tyre pressures and the amount of clobber in the boot and the direction that the steering wheel is being turned and the tyre temperature and the number of passengers and the road surface and the weather and the number of potholes and anything else that might make a difference are the same in both cases (please)
It still seems reasonable to me that worn tyres will give different mph readings to new tyres.
What happens when some of the tyres on the vehicle are worn and others are new I've no idea. _________________ Paul |
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