Speed v Sat Nav

Discussion in 'Tech/Mech General' started by Don Madge, Jul 25, 2010.

  1. Don Madge

    Don Madge Funster

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    Morning all,

    I tried out my new Garmin nuvi 1490T in the Timberland and the car (Renault Megan Scenic Dynamique DCI 130) 06 plate.

    In the van the speedo was about 3 MPH slower than the Sat Nav.

    In the car when travelling with the cruise control set the CC display gave the same reading as the Sat Nav but the main speedo (digital) was reading 1MPH faster than the Sat Nav.

    We don't usually drive to the max speed limit but a moment's inattention could get as a speeding ticket.

    How accurate are the Sat Nav's?

    Can the cars speedo be adjusted?

    What are my options please?

    Safe travelling.

    Don
     
  2. TandH

    TandH Read Only Funster

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    I went to France last year for the first time in my 'van and using the Tom Tom satnav set to Km/hr rather than mph. On the way south to Dover (with the satnav on mph) I was flashed (one flash) on the A1. The satnav speed said I was under the speed limit but the 'van speedo said I was over! Again going round Lyon with the satnav set to KPH I was again flashed once and again the satnav said I was under the speed limit but the speedo said I was over. I immediately ignored the satnav and switched off the speed function. I now only use the 'van speedo and would certainly not trust the satnav to give an accurate speed indication.
    By the way, the flashing was by a speed camera and not somebody in a raincoat!!
     
  3. BobProperty

    BobProperty Read Only Funster

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    This is all in my opinion and as far as I can remember. Speedos tend to be optimistic. It's better to be telling the the driver you are doing 30 when you are really doing 29 than the other way round. I'd also have thought that sat nav would be more accurate than the vehicle speedo, but maybe not by much these days. (Old mechanical speedos were notoriously inaccurate like 10% out). However, the van being 3mph slower than the sat nav sounds wrong. Do you have the original sized wheels and tyres on the van?
     
  4. TandH

    TandH Read Only Funster

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    I am not sure that car satnavs are or need to be that accurate, the mapping software will just put the vehicle location on a line on its map. If you check your location with say a road junction and where the satnav thinks you are there will be a difference - depending on the speed being travelled etc. The satnav software has to make very complex calculations using data which is constantly changing so there will be at least a lag in the display. Car satnavs don't usually give you the accuracy of their positioning whereas handheld units for walking will do so and most of them are only accurate to about 10 metres, which incidently could put you over the edge of a cliff! Satnavs are only advisory and shouldn't be taken as Gospel.
     
  5. Landy lover

    Landy lover Funster

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    A speedometer is fed signals from the gearbox generally ( old system was cable now electronic pulse) which is converted to a speed. Basically an inaccurate way of measuring because the speed varies according to the rolling circumference of the tyres which differs between new and old tyres and also between makes and the tyre pressures - the speedo is set at a level which takes all this into account and tells you a speed with a safe margin to avoid exceeding the speed limit - if you want to know what the variations are you will either have to go on a rolling road machine which will tell you how accurate the speedo is at the current tyres wear and pressure or do the same as the police and have a measured mile which they then check time over distance. The GPS based system is more accurate than speedo. Speed cameras generally flash twice to catch you as two accurately timed photos are taken as a check over the measured road markings. I have been in a traffic jam near a speed camera and it kept on flashing about every two minutes. As I was stationary I just smiled and waited for the next flash :Eeek:
     
  6. dave newell lvs

    dave newell lvs Trader-Vehicle Services

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    The sat nav is as accurate as you're going to get Don. The speedo can legally read up to 10% high but must not read lower than your real speed. Sat nav works by calculating your position in three dimensions (although some people will say its only 2D) based on the time taken for a signal to reach it from the satellites. It needs to "see" at least three satellites to work, the more it can "see" and the further apart they are the more accurate it becomes. Typically sat navs check location once per second and calculate speed by simply measuring how far it has moved in one second (or whatever its frequency of position checking is). This means that the speed display is showing you how fast you were travelling one second ago but this is close enough to be deemed accurate unless you're flying a Tornado. The speedo on the other hand is driven by your wheels, either via a pulse generator at the gearbox output or by the wheel speed sensors fitted for the ABS system. Systems driven from the ABS sensors often take an averaged speed from the ABS ECU to drive the speedo (during cornering all four wheels will be turning at different speeds). As your tyres wear the rolling radius decreases making this method of speed measurement not so accurate.

    Typical discrepancy is 6% over on the speedo in my experience.

    D.
     
  7. GJH

    GJH Funster Life Member

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    I've been told previously that I could rely 100% on a sat nav reading as they are 100% accurate. However, when our TomTom Go 300 reads 70 mph the car speedo is reading 75mph but when our Garmin Nuvi reads 70 mph the car speedo is reading 72/73 mph.

    Sat navs check the speed using computer software acting on the satellite readings mentioned by Dave. Apart from any bugs which there might be in the software it would appear that the program used by TomTom and that used by Garmin produce different results (possibly as a result of rounding differently).

    Sat navs may well be more accurate than a built in speedo but if anyone wants to be sure of not exceeding a limit it would be wise to rely on the latter.

    Graham
     
  8. Glynn

    Glynn Read Only Funster

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    I was informed that the reason for the difference in speeds is due to the fact that sat nav's don't take into account inclines and declines ( up & down gradients ) into account. They only assume you are on a flat road.
     
  9. W18BLA

    W18BLA Funster

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    Sat Nav's are measured by "Time over Distance" and are pretty much 99% accurate this is why all the arm forces around the UK spend so much of there money using GPS! The only problem is line of sight needed so under a bridge it is not going to work or built up areas a struggle. Your speedo will be out by about 2-3% by law every company set's this so they can not be blamed for anything in court if something was link to speed.

    I was pulled over once by a traffic police and he told me i was driving in excess of 34mph on a 30 zone, I told the officer he had better have good reason and evidence of this as i had at the time a GPS tracker which logged every move and speed of the car, He was confident and proceeded to give me a ticket and i took it to court after collecting the data from my tracker and with everything date stamped and times with long and lat info i proved i was infact only doing 29.75mph in a 30 zone and was thrown out of court, It took 4months to win but i did, and it was due to the time over distance i won and the fact he used his speedo and his Vascar which was also proven not to be calabrated that day :BigGrin:...

    So i say GPS if the way forward this is why they are looking at GPS speed limiters fitting in the car so it knows your on a 30 zone and restricts you to 30..

    Thats my waffle anyway :BigGrin:

    Craig
     
  10. Don Madge

    Don Madge Funster

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    After reading all the replies my my original post I realised I was totally confused and decided to take the van out and do a longer road test.

    Fortunately where we live in East Yorks we have a reasonably flat 8 mile length of dual carriageway. I did approximately 500 metres at each speed.

    This was the result

    Van 40 Sat Nav 37

    Van 50 Sat Nav 47

    Van 60 Sat Nav 56

    Van 70 Sat Nav 65

    I did the check for peace of mind and not so I could drive any faster.

    In fact when I was doing 70 in the van I almost "evacuated my bowels" as I don't normally drive at anywhere near that speed.

    So there you have it I'm a happy man again until the next problem pops up.

    Safe travelling.

    Don
     
  11. hilldweller

    hilldweller Funster Life Member

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    Exactly what I would have predicted. I've done this exercise with two GPSs at the same time and both GPSs were absolutely in agreement. I've done it with about 5 over the years and always get the same result.
     
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