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Having joined the ranks of SATNAV enabled drivers I notice that my speed according to GPS is approximately 8% slower that indicated by the cars speedometer. Now I know that the car's speedo should not over indicate by no more than 10% but how accurate is the GPS speed?
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Sat Nav speed is very accurate. After all, it is used for aircraft navigation, guided missles, docking ships and so on, although these are more functions of position than speed! I find that my speedo over reads compared with the sat nav by about 3 mph right thru the range.
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Don't forget that the SatNav speed isn't precise when going up and down hills for the reason you gave about being a function of position rather than speed.
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Hills - while strictly true the deviaion in height is not fast or severe enough to make a difference in a car scenario. Be carefull on bends as sat nav is measuring speed as the crow flies (ie cutting the corner) and can deviate from true speed by 3mph or so, but on the straight its a near accurate as you can get,
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TourVanMan TM < Ex RF >
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After just recently joining the crowd and bought a tomtom, I was supprised at the difference in actual speed. Out of our 3 cars there are differences at the indicated 60mph on the car speedo.
The old Rover 200 does 58 on the tomtom (straight road) whearas the MINI does 56 and my Audi is the worst at 55. Other than the Audi the tomtom stays with the 2/4 mph difference, however the Audi gets further out of sync the faster you go.
Coming from using hand held and nautical GPS systems quite a few years ago to obtain my skippers licence(pleasure craft), I would not doubt that the satnav will be very accurate.
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Being a driver of a Ford, it's interesting to see some sort of "lag or inertia" on the speedo dial when you change speed. Lift off the accelerator and the GPS speed drop but the needle takes time to move. Likewise apply a small amount of acceleration and the needle moves up slowly but the GPS speed rises as you'd expect....
And the reason for including Ford in the post. Turn on the diagnostics for the instrument panel and switch to show digital speed display in MPH... and it's still lower than GPS but reacts a lot quicker like the GPS speed readout.
I find my Mondeo is pretty accurate upto 30mph, quite accurate at say 40-50mph and then the differences open up.
Considering the speedo needle is controlled by the ECU this is quite deliberate. Yes they cannot overread but the needle could move more quickly... should see it flick around when you enter diagnostic mode.
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Cheers all. I had an idea they were accurate but wasn't sure how much. As they are so accurate something crossed mind regarding using the SATNAV indicated speed in challenging a speeding allegation. The obvious problem is difficulty in proving how fast the SATNAV said you were going. What do all you BRs think?
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> SATNAV indicated speed in challenging a speeding allegation
well my assumption is thus.
My speedo is fast according to sat nav, I assume all the police/camera/laser equipment is calibrated and true
SO my speedo is 5 mph fast at 70. the police wont prosecute at anything below speedlimit+10%+2mph
So I drive at an indicated 80mph, knowing my real speed is 75 and I am under the 79mph prosecute zone.
And as I cruise past plod cars at an indicated 80mph, with a glance to them so they know I can see them, without ever being stopped, then Its safe.
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TourVanMan TM < Ex RF >
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I have Via Michelin running on a PDA for my Sat Nav, and I find in the Audi, I am doing exactly 80 when my speedo says I'm doing 85, so quite a bit of optimism there.
However the Audi speedo is bang on between 25 and 35 MPH.
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My Mondeo's speedo under-reads by 8% through the entire speed range according to TomTom
Cheers
DP
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Correction - OVER reads by 8%!
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My old V70 was the best, you didn't even need a Sat Nav to see the over read - its diagnostic mode it gave the true speed read out, rather than the deliberate over read on the speedo needle.
So, as suggested above, I ignored the speedo, followed the diagnostic display (had to convert from Km/H in my head, natch!) and sailed past everyone else doing an indicated 80mph, whilst my true speed was about 74mph, well under anything a camera or copper would do you for.
You can really see the difference when traffic cars blast past everyone. We all see them coming behind us, so we slow down to an indicated 70mph, meanwhile their calibrated display means they're traveling at a real 70mph (76/77 to everyone else), whizzing past everyone else who looks on in envy....
All this must bring into question whether our odometer is recording the real distance the car is covering or its corrected distance according to the speedo display?
At a real 100,000 miles, that could be an extra 8-10k miles showing! Another (unnecessary) service on many cars, money in the bank for the dealers though?
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Does anyone know of an Ipaq program that gives a full screen speed readout? TomTom is ok if you get the passenger to read out the speed, but it's a bit too small to read while driving, I seem to get a lot of reflections on my screen blurring the display.
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GPSDASH2
wimobi.com/gpsdash/overview.ppc2.php
can be configured for digital or analogue full screen speedo
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TourVanMan TM < Ex RF >
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Thanks for that, I'll have a look it.
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Is there something similar for Palm OS as well?
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You can really see the difference when traffic cars blast past everyone. We all see them coming behind us, so we slow down to an indicated 70mph, meanwhile their calibrated display means they're traveling at a real 70mph (76/77 to everyone else), whizzing past everyone else who looks on in envy...
I'm sure when police cars blast past on motorways, they are not limiting themselves to 70mph, more likely doing 100mph+. At 7mph difference in speed they would only dawdle past.
All this must bring into question whether our odometer is recording the real distance the car is covering or its corrected distance according to the speedo display?
I've wondered about that one too. My car odometer is showing 122k miles. Has my car actually done 122k miles or are up to 12000 miles of it ficticious due to the inaccuracy of the speedo?
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Okay, blast past was a poor choice of words. I do actually mean dawdle past, going slightly quicker than everyone else's 70mph. :)
It should be easy to check the odometer reading I suppose. Get a GPS device that shows actual speed, drive for 10 mins at a cruise controlled GPS 60mph and see how much off 6 miles your odo is. I guess we might not like the results though...
Wouldn't make much difference for most of us though? Unless you own the car for a long time. And it's all relative if they all do it, right?
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Would this make clocking the mileage legal? If you moved it back to what it should be, of course.
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Wouldn't that also mean the manufacturer is clocking the mileage against you? I'm sure the lawyers would have fun with that one!
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I've loaded that GPSdash2 software, I haven't got the gps receiver with me but early indications are it's just what I'm looking for. The bonus is the altitude display, it could be quite interesting when I'm out walking.
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bbroomlea, Check the tyre size on the Audi I havd a collegue quiz the speedo accuracy and I took a Trimble Diiferential GPS with me to check his car are I immediately noticed the 50 profile tyres on the standard size alloy rims. End of test. TVM, Hi, the old 10% plus 2 mph is not working any more with 76 mph drivers being done if caught by Laser technology. even Vasar and Pilot systems ahve prosecuted below 78. Yet with all this you would think they would introduce a speedo cal check within the MOT for at the least the cars that are 3 years old. Regards Peter
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"The bonus is the altitude display, it could be quite interesting when I'm out walking."
Memory Map also has altitude display but it is (was??) notoriously inaccurate. I seem to remember reading that this was deliberate - can't remember why but something to do with the data from the satellites. Is the altitude displayed on GPSDash accurate? Anyone know? Suppose it would be easy to check via an OS map and the GPSdash.
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Phil
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"The bonus is the altitude display, it could be quite interesting when I'm out walking." Memory Map also has altitude display but it is (was??) notoriously inaccurate.
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Most normally available GPS systems are inaccurate with regard to altitude, it is interesting to view the altitude display when on a yacht. Should be zero or close to it all the time (tide height varies of course) However it can fluctuate rapidly over a range of 100 feet or so over short periods of time, even with the vessel stationary!
I don't understand the technicalities, but it needs a far more sophisticated, i.e. military , setup to get precise altitude from the GPS system.
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I've wondered about that one too. My car odometer is showing 122k miles. Has my car actually done 122k miles or are up to 12000 miles of it ficticious due to the inaccuracy of the speedo?
I saw an example of this in the early '90s when I took delivery of a new Sierra GLS. After a few weeks of restraint, I took it for a spin on the A14 and got it up to an indicated 130mph - with ease. I started to get suspicious and my local (pathetic) Ford dealership changed the speedo head. There was no change and so they changed it again - and still no change. I noticed also that my journey to work was, over ther same route, taking something like 11.8 miles instead of 9.6 - I forget the exact figures. I then took the car into a smaller local Ford specialist who ran the car alongside another and subsequently discovered that the wrong gear had been used at the 'engine end' of the speedo drive. This was changed and there were no further problems.
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Errmm... got stopped by the police ona quiet eveing on the M5. They asked me what speed I had been doing, I replied, "85". They were quite suprised and agreed, " Yes Sir, 84.9". The 85 was off my GPS with the cruise set.
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Did you get a ticket
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Do the Police use GPS based speedos in their cars?
I was pulled over for overtaking a police car at 40mph on a 40mph dual carriageway!
Copper gets out, so I wind down the window. He asks "do you know what speed you were doing sir?", I say "yes, 40", so he says "well don't do it again" and walks back to his car!!!
I can only assume his speedo was under reading - after all, the police would NEVER drive slowly to hold up traffic!
Incidentally, we have "smiley" speed indicators around here, which seem to be a great way of checking the accuracy of your speedo, or GPSspeed indicator.
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Colin-E
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No I didn't get a ticket - he said that although they normally booked people at 110% + 2mph he didn't consider 85mph in the conditions (12am empty motorway) to be dangerous. In fact he said that he drove about at 85mph (...on motorways).
I was very reasonable and accomodating i.e. not aggressive or arrogant. He probably just wanted to sniff my breath or something although he never even mentioned the possibility that I had been drinking.
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>> He probably just wanted to sniff my breath or something although he never even mentioned the possibility that I had been drinking.
You are almost certainly correct, if they can show you have "committed a moving traffic offence" then they are entitled to stop you, and can then ask you if you have been drinking if they wish. A stop light not working is an equally good excuse. As your behaviour was obviously sober and normal and no sign of whiskey on breath you were allowed to proceed on your way with a warning. Sounds like good policing to me?
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Sat nav speed is only accurate when you are travelling on a flat road the moment you go up or down hill then as far as accuracy is concerned forget it.
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accuracy is concerned forget it
absolutely not. Your speed is being calculated every second. Unless you are changing your rate of desent/ascent in fighter jet proprtions, the change height has NO effect on your caluculated speed.
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TourVanMan TM < Ex RF >
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:Edit: Sorry TVM, but I don't agree with you.
{Rude reply substituted for polite reply - DD}
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The inaccuracy will be caused by the satnav only calculating horizontal speed. This is out from your actual speed by a small fraction when the hill is not too steep, the relation of actual speed to horizontal being speed * cos(theta). When theta is less than 5 degrees or so, cos(theta) approximates to 1, therefore the error will be insignificant.
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I daily drive the length of the M9 - a quiet motorway in my direction of travel, with all its kilometer posts intact. Over 30 miles, my last four cars have shown an odometer accuracy of under 2% whereas every one has shown a speedo reading of 75 or 76 at 70 mph.
I have recently been using a Garmin hill-walkers GPS and have found that unsurprisingly it is in complete agreement with the km posts over this distance.
Moreover, the Garmin gives exactly the same speed reading on my new Civic of 75 at 70mph. (There are fast hillwalkers in Scotland!)
It also shows 34 at 30 mph.
I can only conclude that this over-read is designed in by the manufacturers, and that they could provide an accuracy of 1 or 2% if they wanted to.
However, that's nothing compared with the porkies they tell about speed consumption - but that's another story!
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This speed indication issue raises an interesting question. Is the odometer accurate? After all if my car has really done a genuine 72000m instead of an indicated 80000m that might affect the price significantly?
I have tried this on my 3 series and the odo seems to be pretty much dead on even though the speed seems to show above 8% over at most speeds. Any one else thought this or even better tried it out?
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8% is a lot. Checked my old thre series aganist SWMBO's TomTom found it to be accurate up to around er.....80mph then the innacuracies creep in. Have you got non standard wheels/tyres on it ?
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It's never 'accurate'. Funnily enough it seems most accurate at around 50. Either side it's up to 8% inaccurate but not necessarily linearly through the range - if that makes sense! I imagine that, given the speedo is run off the transmission, that the accuracy will be affected by tyre temperature, pressure and state of wear of the tyre, though clearly not the pattern of inaccuracy through the range of speeds. I haven't gone so far as to start eliminating possible variables...
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Sorry, to answer your original question I have the standard '54 2.0d SE 16" wheels and tyres on it!
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