davenewellhome Posted August 29, 2007 Author Share Posted August 29, 2007 Please let's not take this thread into the discussion of speeding and cameras. The ONLY subject under discussion on this thread is the difference in error between the speedo and odometer on the same vehicle. Now has anyone else checked their speedo/odometer against a GPS system and come up with a different error for each? D. Link to comment Share on other sites More sharing options...
Clive Posted August 29, 2007 Share Posted August 29, 2007 Hi team, We too went to the Caravan Salon at Dussledorf spending a couple of days camping at the Salon site before continuing on to calmer areas in France before comming home this afternoon. Our camper (Merc) speedo reads about 5% faster than the GPS says we are doing, and I believe the GPS. Also the same 5% difference is found in our Toyota 4X4. As I have a nav sat (in the passenger seat) and not a sat nav I cannot comment on distance inaccurancies but would not be suprised to see a similar error in these as well. Interesting show though, did a rapid tour on press day then went back to pick up specific details of interesting items on day 2. By this time we were well and truly knackered! There,s always next year?? We went to Dussledorf in "convoy" with a couple other MMM team members in some motorhomes and all vehicles were fitted with two way radio,s. The other two members did have similar sat nav units and listening to the conversation between the two sat nav users was great fun, especially during the un-scheduled tour around Dussledorf. "Does Clair say next right or next left?" Was one such question. "Dunno" came the reply, "she thinks we have arrived and turned off". What,s wrong with a map I ask? Eventually we aimed for the airport and picked up the Salon signs. Have half a tonne of leaflets and a Mercedez Benz Carrier Bag - Wow! C. Link to comment Share on other sites More sharing options...
davenewellhome Posted August 29, 2007 Author Share Posted August 29, 2007 Thanks Clive (welcome back) for your invaluable input, I'm sure it won't divert the topic of this thread at all :-( D. Link to comment Share on other sites More sharing options...
Mel B Posted August 29, 2007 Share Posted August 29, 2007 Well, at least Clive found something exciting at the show and got a freebie ... rather than witter on about speedo readings .... nowt of interest for you then Davey-boy? ;-) (lol) Link to comment Share on other sites More sharing options...
davenewellhome Posted August 29, 2007 Author Share Posted August 29, 2007 You'll have to wait and read my column in P*H won't you Mel? >:-) I found plenty of interest and if you don't find speedo/odometer readings interesting why are you reading this thread? :-) D. Link to comment Share on other sites More sharing options...
Brambles Posted August 29, 2007 Share Posted August 29, 2007 WHY? The requirements from the EU Directives for a car are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed. A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. So now we can see that all cars speedometers coming of a Production line ar in fact set to be 5% higher as a minimum as required by the regulation. Don't ask why it is specified in km/h for the additional speed tolerance, its to bring Bristish Standards in line with EU Directives. Now the odometer. There are no EU regulations regarding the accuracy of the odometer, it is up to the vehicle manufacturer to specify, but they will normally make this as accurate as technically possible and typically will be +- 2 % or better but could be miles (exuse the pun) out. Account will normally be made for tyre wear by using a mid tread depth of a tyre between max and min wear limits so on a new tyre the reading will actually be slightly lower, but instrument speedo designers I know do not take into account tyre creepage which does occur when driving at speed, up hill etc. This does not answer WHY? Why not make it match the speed? Well why introduce an error when you do not need to. The error on the speedo is because of the EU regulations and also in that during the vehicles life it must remain within the requirements with different makes of tyres being fitted which could have varying tyre circumferences. The odometer will be calibrated to the mean circumferences of the tyres the manufacturer proposes to fit and allowing for wear. Some manufacturers will specify a wide tolerance and some very tight. However one thing normally ignored again is tyre creepage. The reason is because it is very much an unknowm and is dependant on road conditions and driving style. So we have now ended up with a situation wher the indicated speed will be higher than the odometer. The difference between odometers and Sat Nav sytems could be down to many factors, one of which as mentioned above is tyre creepage. Other factors are road irregularities, and how sat nav sytems compute distance as a series of straight lines but as I am not familiar with Sat Nav software I do not really know. What I do know is if I drive 300 miles on a motor way route on a busy day keeping mainy to inside or middle line I will do lets say 300 miles. If I do the same journey at night when roads are quiet and cutting all the corners using the outside lanes on right hand bends (or many when safe to do so) I reduce the distance by 10 to 15 miles. Thats appears to be quite a lot. What i have not figured out is how much is down to more (edited) accelerating when other cars are around, keeping a steadier speed when roads are quiet (edited), and how much is actually the distance saved on the bends. Hope my penny worth helps answer some questions, if not confuse even more. I also dare say the above may have loads of errors and typos for which I make no apology as I am having to rush this reply as I have limited time just now. Jon. Link to comment Share on other sites More sharing options...
Mel B Posted August 29, 2007 Share Posted August 29, 2007 davenewell@home - 2007-08-29 9:00 PM You'll have to wait and read my column in P*H won't you Mel? >:-) I found plenty of interest and if you don't find speedo/odometer readings interesting why are you reading this thread? :-) D. Loosing the will to live ..... I needed to read something totally mind-numbing to take my mind off work ....! (lol) (lol) (lol) (lol) Link to comment Share on other sites More sharing options...
fred grant Posted August 29, 2007 Share Posted August 29, 2007 us in cornwall aint interested in speedos anyway as us cant go very fast due to the millions of grockels this time of year. more seriusly though, i knows us as a speedometer, buts whats a odometer. and where do us find it. is it only on new vans. long way to go to bye a carrier bag clive. was it a old bag says alice? f Link to comment Share on other sites More sharing options...
Old Git Posted August 29, 2007 Share Posted August 29, 2007 fred grant - 2007-08-29 9:24 PM . . . . but whats a odometer. and where do us find it. is it only on new vans. long way to go to bye a carrier bag clive. was it a old bag says alice? f It's a thing for sniffing out red herrings Fred - and I'll bet Dave has got one. Link to comment Share on other sites More sharing options...
Guest caraprof Posted August 29, 2007 Share Posted August 29, 2007 All you people who calculate your journeys on satnav and then compare them to your odometer have far too much time on your hands. I couldn't care less if my satnav gives a slightly different total! What difference does it make to anything anyway?Ps Don't worry about me, I'm just being controversial because I'm bored and there's nothing on the tele. I suppose I could go out and check my sat nav readings. Link to comment Share on other sites More sharing options...
Brambles Posted August 29, 2007 Share Posted August 29, 2007 fred grant - 2007-08-29 9:24 PM . . . . but whats a odometer. and where do us find it. is it only on new vans. long way to go to bye a carrier bag clive. was it a old bag says alice? f from Greek hodometron : hodos, journey + metron, measure. Its the distance travelled indicator on your dashboard, often known as the milometer. Link to comment Share on other sites More sharing options...
Guest caraprof Posted August 29, 2007 Share Posted August 29, 2007 Brambles - 2007-08-29 10:29 PM fred grant - 2007-08-29 9:24 PM . . . . but whats a odometer. and where do us find it. is it only on new vans. long way to go to bye a carrier bag clive. was it a old bag says alice? f from Greek hodometron : hodos, journey + metron, measure. Its the distance travelled indicator on your dashboard, often known as the milometer. And in Europe the mileometer is called a kilometreometer! Link to comment Share on other sites More sharing options...
W3526602 Posted August 30, 2007 Share Posted August 30, 2007 Hi, If I drive my car off a cliff, I will expect my speedo to show a completly inaccurate reading.....same with the odometer.....depending on which pedal I am trying to push through the floor. What would my GPS tell me, assuming I had one? 602 Link to comment Share on other sites More sharing options...
davenewellhome Posted August 30, 2007 Author Share Posted August 30, 2007 Now who's got too much time on his hands Frank? (lol) D. Link to comment Share on other sites More sharing options...
Old Git Posted August 30, 2007 Share Posted August 30, 2007 Hi Dave Thanks to Brambles and others I think we have the answer to your fascinating little poser. Let me try to explain my conclusions as clearly as I can, and see if you agree. (And I'm NOT being cynical here - simply stating what I think are logical and sensible reasons.) I can't think of a situation where an accurate odometer is likely to create a conflict of interests, so there is no reason why it should not be made as accurate as is reasonably possible. The same cannot be said for a speedometer. If they were also made to be as accurate as possible, it would create all sorts of problems for the police and the judiciary. One example should be enough to illustrate the point. All someone would have to do to avoid prosecution for any but a really excessive speeding violation, would be to fit substantially lower profile tyres than those used by the manufacturer. If the speedo was truly accurate, it would then indicate a significantly lower speed than the actual speed of the car, due to the reduced circumference of the tyres. If the speedo was indicating 30mph, the actual speed of the car might be (say) 34mph (can't offer a reasoned extimate, but it would certainly be going faster than 30mph). It would need only one test case in law, which the driver would surely win, and the whole legal system would be undermined and open to frequent and repeated challenge in similar situations. By deliberately calibrating speedos to indicate a higher than actual speed, this takes account of tyre wear, low profile tyres etc., not to mention the inevitable small inaccuracy of the instrument itself. With a built-in 10% (or so) "excess" on the indicated speed, the police and/or prosecution can be pretty sure of their ground if they catch someone driving at more than a small percentage above the statutory speed limit. Hope I have explained this clearly - what do you think? Does this sound like the real reason to you - and let me repeat I am not sniping at the police here. They would be pretty stupid to offer a cast iron escape route wouldn't they! Link to comment Share on other sites More sharing options...
Dave Newell Posted August 30, 2007 Share Posted August 30, 2007 I think you're probably right except for your example. If lower profile tyres were fitted then the speedo would read even higher than true speed. Lower profile tyres will have a smaller circumference and therefore cover less distance per revolution. The speedo is driven by a signal that is directly proportional to wheel rotations and therefore will read higher as the tyre diameter gets smaller. Other than that i agree with your conclusions. Thanks to Brambles for a very clear assessment too. D. Link to comment Share on other sites More sharing options...
Old Git Posted August 30, 2007 Share Posted August 30, 2007 Whoops!!!! That will teach me to be a smartass. Should have put the brain in gear first - is it too late to edit my post so I look less of a plonker?? Link to comment Share on other sites More sharing options...
Clive Posted August 30, 2007 Share Posted August 30, 2007 Low profile tyres should come on bigger rims resulting in the same or similar rolling radius. Yes no ???? Link to comment Share on other sites More sharing options...
Old Git Posted August 30, 2007 Share Posted August 30, 2007 Low profile tyres should come on bigger rims resulting in the same or similar rolling radius. Yes no ???? [/Quote] Hope you are right Clive. I shall not feel quite such a twit then. Regards Link to comment Share on other sites More sharing options...
Brambles Posted August 30, 2007 Share Posted August 30, 2007 Clive, yes and No. You can go for lower profile and increase the rim size, or you can go for lower profile (lower aspect ratio) and increase the tyre width. although that tends to go under fitting 'wider tyres' where you go for say 60% aspect ratio instead of 65%. For those who do not know the aspect ratio is the ratio of the side wall to the width. Many cars have options for different tyre widths to be fitted depending on exact model. The speedo is often calibrated the same for both models, but tyres do have slight differences in circumference. The the Odo and speedo will be pitched so both meet the required accuracy standard. The wider tyre option for normally the upgraded model will have a slightly larger diameter and so the speedo will be made more accurate for that tyre size, the lower spec model having a slightly smaller diameter wheel and therfore will indicate a higher speed v true speed. Hope that makes sense. Jon. Link to comment Share on other sites More sharing options...
mom Posted August 30, 2007 Share Posted August 30, 2007 Dave, despite the battle to get this far!, the answer seems nearly complete, except for the one missing piece of information (for me anyway)... Can anyone explain the mechanics of odometer vs speedometer readings? Are they the same signal feed or independant devices? Are the differences in readout due to post processing in electronics or some other adjustment earlier on? How do modern day speedometer and odometers work? Also, one last... we have a 1992 VW T4. Were these devices advanced enough back then to see the same pattern? We read about 10% over on speedo, but we've not measured odometer. Link to comment Share on other sites More sharing options...
Brambles Posted August 31, 2007 Share Posted August 31, 2007 mom, In effect the speedometer is seperate from the odomoter but within the same unit, sharing the speed signal, or in the case of older mechanical ones sharing the same input shaft. Link to comment Share on other sites More sharing options...
W3526602 Posted August 31, 2007 Share Posted August 31, 2007 Hi, In my days, the odometer was driven by a positive geared device (ok, more complicated than that) so a set number of revolutions of the tyre would notch the digits round by one click. The speedo needle is dragged round by a revolving magnet. As the magnet pases, the needle is moved round the dial. As the magnet moves away, the needle is dragged back by a spring, but is caught by the magnet next time it passes. The faster the magnet returns, the less time there is for the needle to be dragged back, so the needle moves round the dial in an apparently smooth movement. As everything is dependent on Teslas and spring rates, this is not an exact science. I'm amazed that they do it as well as they do. I have also met an early speedo with centrifugal bob-weights which threw out as they turned faster, moving the needle as they did so. I also had a Daimler 2.5 V8 (Inspector Morse Jag lookalike) The tachometer was actually a voltmeter activated by a small generator on top of the engine. Wow! Modern tachos count your ignition impulses. I wonder how they do it on diesels? 602 Link to comment Share on other sites More sharing options...
Old Git Posted August 31, 2007 Share Posted August 31, 2007 W3526602 - 2007-08-31 6:47 AM Modern tachos count your ignition impulses. I wonder how they do it on diesels? 602 That's torn it John - another fascinating question to ponder. I would guess there's an impulse generator in there somewhere, but shall await Dave's or Clive's (et al?) response with interest. If nothing else it keeps the ageing brain cells active!! Link to comment Share on other sites More sharing options...
Brambles Posted August 31, 2007 Share Posted August 31, 2007 Hi, Moderm diesels have engine electronic management systems that have sensors on the engine to detect position and revs. They then supply a signal to the tachometer. On older engines you could either have a sensor on a rotating part, be it the camshaft ,fuel pump shaft or crankshaft, or use the w-terminal on the alternator. It is connected to one of the phases in the alternator before it is rectified and gives you and AC signal proportional to engine speed. Hope that helps, Jon. Link to comment Share on other sites More sharing options...
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