Safe Speed Forums

Yup, I don't even want to think about what maths are going on there.

I assume it will do it over a known number of wheel rotations and average them? If not, I'd be very sceptical. As DCB says, the commercial availability of sat nav position data isn't that good. I used a laboratory-spec. receiver and some software for some car testing a while back. It was good enough to tell you which side of the road I was driving on, but not the sort of accuracy you'd need to measure the circumference of a bike wheel!

when I calibrate my tractor speedo, it knows the rolling radius of the tyre*. Well is does untill you change the load on it anyway.

*that's how it know how far it's been when it has counted the rotations of the wheel.

Funny how it is actually more important to know the exact speed of a vehicle that has a max speed of 30 mph , than a car. The speedo goes to 2 decimal places under 2kph.

Whilst the absolute error can be of the order of several tens of meter the rate of change of error is very slow. So slow that it can be taken as constant between consecutive measurements meaning that the differential error is very low.

Applications such as aircraft blind landing systems which require a high degree of positional accuracy use a differential measurement. A ground station measures its GPS position and , as its real location is very accurately known, is able to calculate the measurement error and transmit it to the aircraft where it is used to correct the local position measurement.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

So how does my satnav get my wheel circumference so spot on then (after about 200 metres)? I'm certain I don't have to ride by any ground stations!

Does 5 metre accuracy really mean that though? What I see on my map is the exact spot where I am surround by an accuracy circle that's wider that the road, but it knows I'm on the road and can even tell which side of the road I am on (it's not locked onto the road on the map BTW).

As DCB mentions, over 200m it will have taken dozens (maybe hundreds depending on how slow you're going) of position readings and will effectively average the errors out. In other words, in the first revolution, it's estimate of your bike wheel's rolling circumference will be lousy. After two revolutions it will be a bit better (I'm assuming it's linked to the bike wheel in some way so that it can count the number of revolutions)? After 20 revolutions it will be better again....and so on.

Because it doesn't matter how big the error is provided it is the same at both measurements. When you subtract position2 from position one the error cancels out ( (x1+e) - (x2+e) = (x1 -x2) however large e is)

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

The accuracy of a GPS is a constant error across a wide area, known as drift. There used to be (not sure if there still are) base stations around that were at a known position and would measure their position as per GPS and then broadcast the drift so that suitably equipped GPS receivers could get a more accurate reading. Few if any GPS devices support this as it's not necessary for navigation, or locating your broken down car or any of the other things consumers use GPS for. It's more useful for things like surveying.

That, Weepej, is why your GPS manages to measure the circumference of your wheel perfectly despite being inaccurate. It knows that you just travelled 20 metres it just has the start and end points wrong.


To the OP I'd warn against using GPS as a replacement for your speedo, what you should do is find a nice flat empty straight road and drive down it at a constant speed after a few seconds of doing a constant speed, compare the two and note the difference. Alternative drive at a constant speed with your GPS at eg. 40mph and note what your speedo is reading.

Be aware that the calibration will change over time, as your tyres wear out your speedo will overread more, which isn't a huge issue, however when you replace your tyres if you rely on the old calibration you'll end up a few MPH over the speed limit and suddenly you've gone from being a good safe obedient driver to an OMG! child killer, so watch out for that.


Eventually you'll get used to doing the GPS calibration check each time a suitable road comes up, these days I can do it without hardly paying any attention, so I just do it in my head a few times on each trip and thus I can stick to the speed limit.


Oh and the 10%+2 thing on cameras is mainly to protect the camera operators from having their calibration called into question. If they started doing people for 71mph in a 70 you could easily argue that their device may have drifted slightly in calibration since it was last checked, but not prosecuting until 79mph means they can argue, so what, so you were only doing 78, you were still speeding. If you drive to 10%+2 and thus at 78 on the motorway and get caught by an ever so slightly out camera, you risk getting done.

Alternatively one can simlpy ignore the faulty instrument supplied by the manufacturer and use the GPS as one's primary speedometer. I find that this has the further advantage of requiring less eye movement to check the speed as the GPS is mounted above the dash

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Bad idea. The GPS will lag behind by a few seconds and even then can only be relied upon on a flat roat doing a constant speed. On hills it will read artifically low.

You can get little HUD units that you wire into the speed signal going to your ECU if you really want a speedo above the dash, some of them will even let you set the calibration yourself, which gives you the best of both worlds. I have one in my car, but then I need it because the main speedo is in km/h only.