speedo reading versus gps

[carolh]

Which you would say was more accurate, the speedo reading or the speed shown on the satnav readout? There is a difference of 5 mph between the motorhome ie - mh 60mph - satnav 55mph - any vehicle I have driven and used the satnav there is always a difference! just curious carol

[Derek Uzzell]

As a general rule speedometers over-read (sometimes by a considerable amount), so it seems reasonable to assume that (because a satnav system needs to be accurate to be any use) the satnav speed reading will be correct. A speedo 'over-read' of 5mph at 60mph sounds perfectly normal.

[Dave Newell]

The legal requirement for speedos is that they may read up to 10% higher but not lower than your actual speed so 5MPH over at 55MPH true speed is just within the limits. As Derek says the satnav reading will be accurate. The average speedo error is 6% over in my experience but I have heard of several people having problems with excessive over read usually caused by the wrong wheels being fitted i.e. 15" instead of 16". D.

[enodreven]

Hi, The Sat Nav is speed as the crow fly's, so up or down hills it will wrong ?? and is therefore unacceptable to use for measuring road speed Hope this helps ??

[Dave Newell]

Sorry Brian but I have to disagree with you. The sat nav systems can pinpoint your vehicle to within 15 metres typically, sometimes better, sometimes worse, but this is three dimensionally so it willtake into account uphill and downhill fluctuations to some extent. The sat nav displayed speed is also averaged so it gets more accurate over longer distances. Overall it's far more accurate than the built in speedo. My one concern is why so many people feel it is so important to have such accuracy in the displayed speed. Until sat nav gave us accurate speed readings we all carried on in blissful ignorance of the fact that our speedos (and odometers) told lies, albeit small ones. D.

[carolh]

dave How true - my speedo says 60mph, I think am driving to the speed limit and I am happy at this speed, I don't feel that I am holding traffic up - anyone who wants to exceed the limit and pass me - go ahead! But now I know my 'true' speed is only 55mph - will I be tempted to go to 65mph on the speedo - I don't think so, but i will be aware that I am maybe dawdling:) I drive at this speed because for my mh it is economical and I am comfortable with it. On a dual carriageway on a fine day, downhill with a following wind - then maybe 70pmh on my speedo for short distances, but not mile after mile. Knowing the true speed won't change my driving habits:) thanks for the replies Carol

[Frank]

[QUOTE]enodreven - 2006-06-22 4:26 PM Hi, The Sat Nav is speed as the crow fly's, so up or down hills it will wrong ?? and is therefore unacceptable to use for measuring road speed Hope this helps ?? [/QUOTE] This is not the case since you are obtaining an average of all the readings from the satellites that are plotting position at any one time. This would only be true if measured from a single position, i.e. directly above. The speed indicated from satnav is likely to be more accurate than your speedometer.

[Frank]

[QUOTE]carolh - 2006-06-22 10:18 PM dave How true - my speedo says 60mph, I think am driving to the speed limit and I am happy at this speed, I don't feel that I am holding traffic up - anyone who wants to exceed the limit and pass me - go ahead! Carol[/QUOTE] A good point, not much use in being able to calculate speed to within a couple of miles per hour when we are driving motor homes! Regards Frank

[enodreven]

Hi, Only quoting Honest John in the Telegraph ?? i will try it next time that i am on a long hill, to see if the difference between it and the speedo changes. Also it does seem funny that nearly all of the people who have spoken have about the same discrepancy 5 mph ?? I do agree with you Dave regarding the speed ? element, also just in case HJ is correct be careful near speed camaras ?? Brian

[Dave Newell]

Hi Brian, I've heard about this "Honest John" chap before and he was wrong then as well! 3-5 MPH discrepancy is typical at around 55-60 MPH indicated speed and falls within the 10% (there is actually a complicated formula for calculating the allowable error but in essence it's 10%) allowable error. The error factor is why I always use the speedo and never the sat nav as it doesn't allow any safe margin. If I'm doing a speedo indicated 50MPH in a 50 MPH limited area then I know I've got a few MPH spare to allow for gaining speed on downhill stretches. If I'm doing 50 MPH sat nav indicated in the same zone then I've got no margin for error. Travelling at exactly the allowed limit is of no importance to me whatsoever whereas travelling at a comfortable and legal speed is. D.

[colin]

I seem to remember reading that speedo's where allowed to be 10% out up to 30mph then 3mph for speeds above this, but I have no idea if this is true. I also remember reading that you could check speedo by timing distance between seveval motorway marker posts. If only I could remember how far apart they are.

[docted]

Colin You could always get out with a ruler and measure. But a far afer method is to accept 100 metres, and use the plates with the numbers on them to measure your distance travelled. There actually , suprisingly for a government dept a logic to them.

[Mel E]

I'm afraid 'Honest John' is right. The SatNav system measures how far you have travelled between each 'fix' it takes and uses this to work out your speed. An aviation SatNav will show your approximate height above sea level, but vehicle satnavs do not bother with such information. In any case, SatNavs have no knowledge of the terrain over which you are travelling, which they would clearly need in order to correct for up or down hill. They basically assume a flat earth at sea level. As long as you are travelling on level ground, they will give a very accurate speed readout (any correction for the fact that you are on the Spanish plateau at, say, 3,000 feet so your journey from A to B is round a slightly larger circle is so small as to be ignorable.) So up and down hill they give the flat earth speed; you are travelling a greater distance (the hypotenuse of a triangle) so you actual speed will be greater than that shown.

[Dave Newell]

Hi Mel, I can't speak for all of the vehicle sat navs in use but the VDO Dayton system definitely gives altitude. As you rightly say they take regular "fixes" and calculate speed from the distance travelled in the time between fixes but as they can all handle at least 8 different satellite signals simultaneously I would expect the speed readout to be completely accurate. 3 satellite reception is required to give 2 dimensional positioning but anything over 4 satellites will give 3 dimensional positioning. D.

[Derek Uzzell]

It might be worth referring to an "Interchange" letter in May 2006 MMM (page 244). It seems from George Collings' response that a substantial speedometer 'over-reading' error (in the letter-writer's case 10mph at 60mph) can be legally permissible. If we can overlook technical GPS-related variables involving altitude changes, satellite numbers, sat-nav models etc, then using the sat-nav reading (on, say, a straight, flat stretch of motorway) has got to be the simplest means of confirming the accuracy of your vehicle's speedometer.

[Frank]

[QUOTE]Mel E - 2006-06-26 1:12 AM In any case, SatNavs have no knowledge of the terrain over which you are travelling, which they would clearly need in order to correct for up or down hill. They basically assume a flat earth at sea level. So up and down hill they give the flat earth speed; you are travelling a greater distance (the hypotenuse of a triangle) so you actual speed will be greater than that shown.[/QUOTE] Hi Mel I wasn’t even aware that there were any systems using less than four satellites, it would be very strange if that is the case but it would not be possible for altitude calculations in such a system. I agree that there will be an apparent shortening of distance travelled whilst on an incline but this would only occur if viewed from a fixed point above the incline which is what I said in my previous post. GPS systems use three satellites for position fix (triangulation); they require at least four satellites to provide altitude calculations and these are done simultaneously with speed and direction thus allowing for inclination. The only time errors may be produced is if you were unlucky enough to be receiving signals from a group of satellites clustered close to each other. This is much more unlikely these days since satellites (Navstar) have been placed in much better orbits than previously and the new European system (Galileo) due in about three years time will be even more accurate. Additionally, as far as I am aware, most GPS’s use several channels so any number of channels above the four required for the initial reading will be used to augment these and provide greater accuracy. But you would clearly need to look at the specifications of each GPS to determine what it is able to receive. I also use my GPS for the mountains where I am invariably either climbing or descending, if the system could not account for this then the time and distance calculations as well as height gained or lost would be woefully inaccurate but it is not like this in practice when compared to normal navigational techniques. Regards Frank

[enodreven]

Hi, Take a look at this link it may help ? also isn't there something about the accuracy of speedo's for road use, being acceptable if they read upto a certain % over the speed being traveled but no allowance is acceptable for reading under ?? http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15519597&dopt=Abstract

[David Dwight]

Mel E, The GPS Microsoft autoroute I use give altitude readings. David

[colin]

Go to 'gpsinformation.net/main/gpsspeed.htm' n.b. no www, this will give you an answer, but what it does't mention is gps can struggle to get accurate position, and therefore speed, when it is 'swapping' satellites such as when driving throu towns with high buildings.