kelly58 Posted April 13, 2007 Share Posted April 13, 2007 Whilst driving at a constant 50 mph my tom tom shows a speed reading of 46mph and obviously less aat lower mph which do I believe speedo or tom tom ? Link to comment Share on other sites More sharing options...
Brian Kirby Posted April 13, 2007 Share Posted April 13, 2007 kelly58 - 2007-04-13 6:36 PM Whilst driving at a constant 50 mph my tom tom shows a speed reading of 46mph and obviously less aat lower mph which do I believe speedo or tom tom ? Believe the speedometer. They are usually "fast", so you don't get booked for speeding. Your Tom-Tom will only sample your speed periodically, will be interrupted by overhanging trees etc, and may be influenced by how many satellites it can "see". Trying to use it as your speed reference in real time may well get you booked! Link to comment Share on other sites More sharing options...
Terrytraveller Posted April 13, 2007 Share Posted April 13, 2007 Hi Kelly, Sounds about right to me. My Mio tells me I am travelling at 26 mph when the speedo is telling me I am travelling at 30 mph. When passing by one of those static speed display LED monitors in the village of Blunsdon, the monitor says slow down to 30 mph, while the speedo is indicating exactly 30mph. At 70 mph on the speedo, the satnav is indicating a speed of 62 mph, and my wife is telling me to slow down. Brian there was no tree's in the way either and I am holding a steady speed, the satnav is supposed to sample every second and normally has between 7 and 11 satellites acquired, so will be pretty accurate. Having said that, the elevation isn't always right - so what do you think of that, anyone else found the elevation not reported accurately? Regards Confused 8-) Link to comment Share on other sites More sharing options...
Randonneur Posted April 13, 2007 Share Posted April 13, 2007 We have the same problem with our Mio. The satnav keeps saying that we are exceeding the speed limit but the speedo says not. Mind you when we are out in the motorhome we have it set up as a lorry, so that could be one of the problems. Link to comment Share on other sites More sharing options...
Guest starspirit Posted April 13, 2007 Share Posted April 13, 2007 Most if not all speedos are up to 10% fast because the government allows them to be on the basis that it is another way of keeping speeds down at no cost to them. The technology has existed for yonks to calculate speed over ground to very precise levels yet car makers choose not to use it. Given the UK's well publicised speed limit tolerance of 10% + 1 mph it is probably safe to use the sat nav as a speedo on long fairly even speed sections of road - such as keeping it on 50mph in a 50mph motorway road works section. Where it falls down is whilst accelerating and braking when the time lag can mislead you quite badly. Furthermore in those circumstances having to look at two instruments simultaneously whilst in the vicinity of a junction or roadworks is positively dangerous. Be a little more circumspect abroad where tolerance levels as such do not exist and 50 means 50. However, I do find the ability to show a GPS in mph or kph is very useful abroad Link to comment Share on other sites More sharing options...
Terrytraveller Posted April 13, 2007 Share Posted April 13, 2007 Hi Sylvia, Your speedo seems to be slow, my speedo is fast, are you sure you have got the problem the right way round. I have checked out two satnavs against the speedo with the same results, i.e. speedo displays 30mph and both satnavs tell me I am going slower. Speedo's should never be under calculating your actual speed as it could lead to obvious problems keeping to the speed limits. I wonder if you have changed the speedo, wheel size or gearbox/final drive or even have the wrong bits fitted from new? Regards Terry Link to comment Share on other sites More sharing options...
Randonneur Posted April 13, 2007 Share Posted April 13, 2007 No, no changes to speedo, only bought the van in Oct 2006 although its a 1998. It doesn't make must difference to us as my other half is a steady 90 KmPH (about 56mph) driver. I've not been allowed behind the wheel yet. Link to comment Share on other sites More sharing options...
Guest starspirit Posted April 13, 2007 Share Posted April 13, 2007 It is illegal for a speedo to under read even by 1 mph, which is another reason why they are all so optimistic - to prevent the manufacturer being held to account for supplying the inaccurate bit of kit euphemistically called a speedometer. Another reason is that the more miles the odometer shows the more mpg we all think we are getting. And another is that a 12000 mile service also falls due after 10800 miles. No wonder they don't want them accurate! Link to comment Share on other sites More sharing options...
Dave Newell Posted April 14, 2007 Share Posted April 14, 2007 Speedos are allowed to read up to 10% over but not under at all. The reason for this has nothing to do with government wanting us to drive within set speed limits but it harks back to earlier times when speedos were mechanical and not so accurate. It also allows for tyre diameter variation due to wear. Always work to your vehicle's speedo as it gives you a percentage safety margin against being caught for speeding. While the UK is supposedly giving you 10%+1MPH allowance on speed limits I beleive the continentals, French in particular do not give any allowance, 1MPH over = a fine. The sat nav is far more accurate overall but can suffer from time lag. D. Link to comment Share on other sites More sharing options...
Basil Posted April 14, 2007 Share Posted April 14, 2007 I have checked all my speedos against my sat nav and all but one, which is 100% accurate, read under the correct speed. Interestingly contrary to what Dave has just stated the one that reads completely accurate is a mechanical one of the others all but one are electronic. Having said that the one that undereads the most is the Boxers which reads 70mph when the sat nav says 64mph and is mechanical. I normally use the speedo for my speed checks but if cruising on cruise control I may set it to the sat nav speed as it more accurately controls the speed than my foot. Bas Link to comment Share on other sites More sharing options...
Dave Newell Posted April 14, 2007 Share Posted April 14, 2007 Bas, I think you mean over-read don't you? If it says you are travelling faster than you actually are that is an over-read. D. Link to comment Share on other sites More sharing options...
Basil Posted April 14, 2007 Share Posted April 14, 2007 Quite right Dave don't know what I was thinking there. To clarify the speedos were saying I was going faster than the sat nav said I was. Is that right now? :$ :$ :$ Bas Link to comment Share on other sites More sharing options...
tonyishuk Posted April 14, 2007 Share Posted April 14, 2007 Having had a Caterham, where the speedo was wildly inaccurate in anycase. (Honest) Changing tyres made quite a difference to the rev to speed ratio. Best way to calibrate was using a steady speed and a good GPS system to read the speed and mark the speedo. and /or use a bike speedo and a magnetic reader on the front wheel. Mind you over 99.9 MPH you had to estiamte the speed a little :D Point being, Wheel diameters have quite abearing on the accuracy of the speedo. Mabe the tyres have a different circumerence to the originals. Link to comment Share on other sites More sharing options...
g4oip Posted April 14, 2007 Share Posted April 14, 2007 Have checked both my vehicles with my garmin and both read faster than the garmin, but, lorries which I used to think were speeding more than their limiters were supposed to allow are exactly right as per. Garmin but read 60 mph on the odometer of Iveco Daily and Fiat MH. So providing you are in the clear from trees etc. you can see what your speed really is. Alec Link to comment Share on other sites More sharing options...
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