Melenter Posted October 24, 2016 Share Posted October 24, 2016 Which is the most accurate speedo your vehicles or the satnav? *-) Link to comment Share on other sites More sharing options...
snowie Posted October 24, 2016 Share Posted October 24, 2016 I always trust the sat-nav Regards Alan Link to comment Share on other sites More sharing options...
Tracker Posted October 24, 2016 Share Posted October 24, 2016 A sat nav is reactive and unless speed is constant is always behind the true speed. A long stretch on a level motorway at a set speed on cruise control allows one to compare the two. The mile or kilometre posts on a motorway also let one time the distance if needs be. From the checks I have carried out in the past if for no other reason than it helps to pass the time on long hauls I reckon most speedos on vans are around 10% fast, and so possibly is the odometer which means that servicing mileage is also 10% less than it should be? Or maybe I am just cynical? Link to comment Share on other sites More sharing options...
Dave225 Posted October 24, 2016 Share Posted October 24, 2016 I have heard that all makers put speedos in vehicles that are slightly optimistic. In other words if it shows 30 then you are probably doing 28/29 mph. It is possibly to allow them to avoid claims from owners who have been caught speeding that it was the speedo's fault. Link to comment Share on other sites More sharing options...
aandy Posted October 24, 2016 Share Posted October 24, 2016 The speedo in my Ducato consistently reads just under 10% short of what the satnav shows. I am confident that it is the speedo that is wrong as I have tried two different satnavs with the same result, and where I am able to maintain a steady pace approaching a roadside speed indicator it always agrees with the satnav. I know speedos are generally a bit optimistic, but that seems rather a lot. It is obviously the speedo itself that is at fault, rather than the sender, as the odometer is within 1% of the satnav. Link to comment Share on other sites More sharing options...
Den Posted October 24, 2016 Share Posted October 24, 2016 The reason speedos are set with caution is. Tyre wear. Lorry, coach and minibuses have there Tachos calibrated with heavily worn tyres. They can then drive at the top end of the restricted speed. Link to comment Share on other sites More sharing options...
Steve Drew Posted October 25, 2016 Share Posted October 25, 2016 aandy - 2016-10-24 8:33 PM The speedo in my Ducato consistently reads just under 10% short of what the satnav shows. I am confident that it is the speedo that is wrong as I have tried two different satnavs with the same result, and where I am able to maintain a steady pace approaching a roadside speed indicator it always agrees with the satnav. I know speedos are generally a bit optimistic, but that seems rather a lot. It is obviously the speedo itself that is at fault, rather than the sender, as the odometer is within 1% of the satnav. Our Ducato Speedo is also just under 10% out and that is with fairly new tyres as they wear its going to get closer toor over 10% as the rolling circumference changes. Ours has 15 inch wheels our friends Ducato has 16 inch wheels his is far more accurate it would seem that maybe Fiat may be fitting one speedo on all models. Link to comment Share on other sites More sharing options...
Muswell Posted October 25, 2016 Share Posted October 25, 2016 The regulations state that a speedo must not show less than the actual speed, nor more than 110% of actual speed plus 6.25mph....so at 50mph your speedo could legally show 61.25mph. Link to comment Share on other sites More sharing options...
Guest pelmetman Posted October 25, 2016 Share Posted October 25, 2016 Every modern vehicle I've driven the speedo has been out by almost 10% when compared to the sat nav.... Horace's cable driven speedo is 2% over :D ....... Link to comment Share on other sites More sharing options...
Steve928 Posted October 25, 2016 Share Posted October 25, 2016 It's possible to correct the speedo using the manufacturer's diagnostics package or equivalent i.e. MultiEcuScan for Fiat or PP2000/Lexia/Diagbox for PSA. Hidden in all the pages of parameters is a 'wheel diameter' value which is used to convert wheel rotations to speed (and mileage, hence fuel consumption calculations are over-optimistic too..) and which is set low to produce the desired over-reading. I've corrected mine and it's now accurate. Link to comment Share on other sites More sharing options...
lennyhb Posted October 25, 2016 Share Posted October 25, 2016 Tracker - 2016-10-24 8:07 PM From the checks I have carried out in the past if for no other reason than it helps to pass the time on long hauls I reckon most speedos on vans are around 10% fast, and so possibly is the odometer which means that servicing mileage is also 10% less than it should be? Or maybe I am just cynical? Carried out checks on my last van and the speedo was 10-11% over but the odometer spot on. Never checked the odometer on current van but speedo 9% over. Link to comment Share on other sites More sharing options...
BnJ Posted October 25, 2016 Share Posted October 25, 2016 GPS calculated speeds have inherent problems and for those who want to know the reasons I go into these in the paragraph below. However with that knowledge I still believe that in general and whilst on the move in a good signal receiving situation, the GPS speed will be (in general) more accurate than our vehicles speedometer. The techie stuff: The GPS receivers used in our cars and vans are simple single frequency receivers and as such the best positional accuracy we can expect from them is around is + or - 5 metres. (More accurate real time kinetic dual frequency receivers as used by surveyors and navigators have a positional accuracy of up to 2 cms.) Positional accuracy is required to calculate speed of course (the time taken to travel the distance between 2 points). The accuracy of the time signal in a GPS receiver is excellent as its based on an atomic clock in the satellites and the ground monitoring stations, however the weak point as I said above is the positional accuracy of the 2 points used to measure the distance. Now it might well be that the error in those 2 points is exactly the same, so the measured distance between them is good, but it might also be that if the 1st point has an error of + 5 metres and if the 2nd point has an error of – 5 metres there there will be a 10 metre error in the distance. Given the epoch between the GPS signals that fix the 2 points might be short, a 10 metre error in the distance could make a very big error in the calculated speed. That is of course an extreme situation but you get my point. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 26, 2016 Share Posted October 26, 2016 French roads often have ‘radar’ speed-reading signs that (presumably) are accurate. I’ve always found that the kmh readout on these equipments always tallies with the kmh readout of my Garmin satnav. Cross-checking the speedometer with the sat-nav and radar signs indicates that the speedometer overeads by the usual 10% or so at 90kmh (not that the speedometer is actually readable in the first place!) Link to comment Share on other sites More sharing options...
tonyishuk Posted October 26, 2016 Share Posted October 26, 2016 Some while ago Caterham 7 owners had pedal bike speedometers fitted to their cars, because the Caterham speedos were wildly inaccurate. There was much debate about wheel / tyre sizes, out of balance because the only place to glue the magnet for the sensor was on the wheel. GPS came along and it was found that GPS was an acceptable alternative as the readings were very similar. whilst on the road, but not acceptable for MOT. Oh back to the days with flies in yer eyes (and teeth), and smell of burning rubber, brakes and engine oil ! Rgds . Link to comment Share on other sites More sharing options...
aandy Posted October 26, 2016 Share Posted October 26, 2016 How about one of these as an alternative? http://l7.alamy.com/zooms/4c4e2a64ea9d4457b954b462a8908b90/primitive-air-speed-indicator-on-the-wing-strut-of-a-de-havilland-barxn1.jpg Link to comment Share on other sites More sharing options...
Melenter Posted October 27, 2016 Author Share Posted October 27, 2016 Thanks for all your replies. The main reason I asked the question as Derek pointed out is the crap speedo in the ducato ( unable to read the damn thing, so much easier reading the sat sav happy travels :-D Link to comment Share on other sites More sharing options...
sshortcircuit Posted October 27, 2016 Share Posted October 27, 2016 I have always had the understanding that the 'speed' was shown 10% above actual but never considered that the "mileometer" was also set at 10% less as has been suggested which would suggest that a vehicle has done 10% more than shown on the speedo??? I also understood that GPS measures your horizontal speed and you could actually have a surface speed in excess of that when going down an incline which is the speed measured by police radar. Whilst I am aware of my GPS speed I stick to the vehicle speedo. Link to comment Share on other sites More sharing options...
keninpalamos Posted October 27, 2016 Share Posted October 27, 2016 Den - 2016-10-24 9:07 PM The reason speedos are set with caution is. Tyre wear. Lorry, coach and minibuses have there Tachos calibrated with heavily worn tyres. They can then drive at the top end of the restricted speed. ? Lorry, coach and minibuses have their Tachographs calibrated with what ever tyres are on the vehicle at the time which have to comply with the DOT rules and regulations at the time. I think the term "heavily worn" could be miss leading and I, over many years have not found your suggestion to be the case. Former DOT commercial MOT test inspector, mechanic, etc Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 27, 2016 Share Posted October 27, 2016 Although (as Muswell advised earlier) there are legal requirements relating to the accuracy of a vehicle’s speedometer, as far as I’m aware similar legal requirements do not apply to odometer accuracy. In fact, I’m pretty sure there’s no legal requirement (in the UK at least) that a vehicle be fitted with an odometer or, even when it has one, that it be in working order. On-line 'received wisdom' suggests that the accuracy of a vehicle’s odometer tends to be proportional to the accuracy of its speedometer - so if a vehicle’s speedometer over-reads significantly, there’s a good chance that the odometer will too. Steve928 has said that it’s possible with the right equipment to ‘tweak’ a Boxer/Ducato X290’s software so that the speedometer shows the vehicle’s true speed, and there’s every reason to think that his subsequent comment is correct that doing this will have a knock-on effect on other calculations based on wheel-rotation data. There’s a useful piece on GPS-based speedometers here: http://gpssystems.net/accurate-gpsbased-speedometer/ As far as Ducato X250/X290 speedometers are concerned, my Rapido is LHD, so its speedometer is on the left side of the instrument-cluster and near-ish to the left cab-window. The speedometer-face is also kmh-only (Yeah, yeah - I know it’s not UK-legal) but it’s still not easy to read any of the instruments in daylight. I’ve front-lit the instrument-cluster with leds, which is much more effective than Fiat’s ploy of having the cluster’s back-lighting come on with the ignition, but I much prefer to use my Garmin sat-nav’s speed readout that I can switch between kmh and mph. I’ve no idea how one is expected to read a kmh speed on a RHD Ducato when the speedometer-face has dual mph/kmh scales. Link to comment Share on other sites More sharing options...
aandy Posted October 27, 2016 Share Posted October 27, 2016 " if a vehicle’s speedometer over-reads significantly, there’s a good chance that the odometer will too" Not sure whether my experience is typical, but I checked the odometer over a couple of hundred miles and found it to be accurate, despite the speedo being nearly 10% out. I also checked the fuel consumption readout over 2,000 miles and was surprised to find that it was within 0.5MPG of my manual calculation. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 28, 2016 Share Posted October 28, 2016 Andy There is a good deal of on-line discussion about odometer accuracy https://www.google.co.uk/?gws_rd=ssl#q=odometer+accuracy and the Wikipedia entry https://en.wikipedia.org/wiki/Odometer says "Odometer errors are typically proportional to speedometer errors.” If a vehicle’s speedometer and odometer are ‘electronic’ and speed and distance-covered are both based on the same wheel-rotation data, it would be logical to expect the respective speed and distance-covered calculations to march in step. Steve928 has been able to recalibrate his Peugeot Boxer X290 motorhome's speedometer by adjusting the 'wheel-diameter' datum in the vehicle’s software. As he says, the vehicle’s speed will be calculated based on the number of wheel-rotations taking place in a given time and the wheel-diameter datum and, if the wheel-diameter datum is altered, the speed calculation will be affected. Although it would be practicable with ‘mechanical’ instrumentation to gear the speedometer and odometer dfferently so that the speedometer was optimistic but the odometer was not, it seems more likely with an electronic system that, if the wheel-diameter datum is changed, this will impact on the odometer. For example, a motorhome based on a Boxer/Ducato X290 ‘light’ chassis will have 15”-diameter wheels and 215/70 R15 tyres as standard, but there’s an option to have 16”-diameter wheels and 225/75 R16 tyres. Whichever wheel/tyre combination is fitted, the vehicle’s 'wheel-diameter' datum will need to match so that the appropriate speed and odometer calculations can be made. It would, of course, be technically possible to have separate wheel-diameter values - one used for speed calculations and another used for odometer calculations - but it’s much more likely that there’ll be a single datum that will be used in all calculations that include a wheel-rotation factor. Alternatively, when using the wheel-diameter datum in calculations, the vehicle’s software could deliberately enhance the speedometer readout so that it never under-reads, but leave odometer-related calculations alone. As your motorhome has a speedometer that significantly over-reads, but its odometer and fuel-consumption readings are accurate, perhaps that’s what’s happening. But if you followed Steve’s lead and adjusted the wheel-diameter datum to remove your speedometer’s optimism, it’s probable that this would affect the accuracy of the odometer and fuel-consumption calculations. Link to comment Share on other sites More sharing options...
StuartO Posted October 28, 2016 Share Posted October 28, 2016 Once upon a time police traffic cars used to have special speedometers which could be calibrated daily, to ensure an accurate basis for prosecution. As far as I know that was done by installing a special instrument rather than relying on the car's own speedo. Does anyone know how they do it these days? Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 28, 2016 Share Posted October 28, 2016 https://www.google.co.uk/?gws_rd=ssl#q=police+car+speedometer+calibration Link to comment Share on other sites More sharing options...
aandy Posted October 28, 2016 Share Posted October 28, 2016 Derek, I understand what you're saying about altering the 'gearing'. I don't know how electronic instruments work (or how anything electronic works), but in a mechanical system a fault in the magnet/spring assembly that drives the needle would not affect the odometer. I assume there is some electronic equivalent and that that is what is causing my speedo to over read. Stuart, I imagine that as the police now have all sorts of electronic gizmos to measure speed and video their targets the car's speedometer is irrelevant. Link to comment Share on other sites More sharing options...
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