GPS accuracy vs. speedo

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GPS accuracy vs. speedo

FIATasafiddle

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Can anyone help me understand the difference between how sat nav and a speedo measures speed? I mean obviously I get that one uses satellites and the other uses data from the car, but what causes them to be different - and are sat navs always more accurate? Do speedos always over estimate your speed in your experience?

Thanks:D
 
Speedos used to tap off the gearbox, gearing would turn a flexi shaft which would be translated at the clock into a moving pointer (clockers used to put a drill to this shaft to wind the clocks round to where they wanted them!)

I imagine modern speedos either use the same tech or count pulses from a magnet on a rotating shaft (similar to how a speedo on a bicycle works?), but not having pulled a modern speedo, I know not how they work.

A satnav has to take signals from at least 3 (?) sats to triangulate its position on the planet. As you move along, it re-triangulates its position, calculates the distance from the last point, how long it took to cover that distance & display this on the screen. These calculations are, of course, done extremely quickly. However, the faster you drive, the less accurate the speed reading.
This 'lag' is one of the reasons why many take wrong turnings.
On a roundabout, for example, your exit is ahead on the screen but you can be alongside it. If it says "take the third exit", best to count the exits rather than rely on the screen route to refresh.

The digital tacho on my bus always seems to correspond closely to the speedo on the dash. (maybe because both utilise the same method to record speed? - although the tacho doesn't have a driveshaft cable going into it)
If you were to change size of wheels and/or tyres, your speedo will no longer be accurate
 
Speedos are allowed to over-read but not under-read, as a result most modern cars have a 10% increase programmed into the software to display a faster speed.

As a result the GPS will read closer to the actual speed being driven if at a constant.

Not looking at my GPS.
Last week I had to drive to southampton, the bus is fitted with a limiter so my max speed was 100kph (a smidge over 62mph).
On the long stretches of motorway, my tacho read a steady 100kph, the needle on the speedo settled just over the 60 - yet my tomtom was all over the place - up to 69mph in places yet the limiter prevents the vehicle from going more than 100kph.
I also have a very expensive gps system for walkers which will record all data from a walk so it can be reviewed later, there are times when I look to have been running along, there are times when I am resting yet the gps has me moving around up to 30 yards from the point where I am resting, the trace looking like some sort of manic scrawl.
 
GPS is set by 3-4 sat's they judge you speed by calculating distance over time, giving you a much more accurate speed,(taking in to account processing lag on your unit) on you car as said before your speedo can be over but not under. So you can't be doing 75 and you speedo displaying 71, as if you where done for speeding you could potentially sue the manufacturer as you believed you where doing the speed limit.
 
The difference.

the speedo in your car is taken off the output shaft of the gearbox the rotation of the speed sensor sets up an electronic analogue pulse which is sent to the ECU for encoding into a digital signal sent out to all the cars systems including the speedo which takes the digital information and turns it into something that can be read, a moving needle or a digital display, essentially the more pulses (and the greater the amplitude of the signal) the faster the car is going, the software in the car needs to be calibrated to take into account the tyre size and the step down ratio of the differential, with usually a built in error of margin for tyre wear etc, they are a lot more accurate than they used to be.

GPS systems use a network of highly accurate atomic clocks in satellites which send out time information location and identity the GPS needs at least 4 satellites as its triangulation in 3 dimensions. all the calculations are done in the GPS unit, this then sets your location
speed is calculated from the change in your locations, the more satellites the more accurate the location in some cases as close to a metre. and the faster the gps unit can process the more accurate the speed, for example my current TomTom has more processing power than the PC I owned in 2001.
But the GPS signals are weak so any interference, being inside, a heated windscreen, weather conditions etc all cause problems which can reduce their accuracy.

essentially a GPS calculated speed is more accurate as long as the signal is good generally if my satnav says i'm doing 60mph the cars speedometer will say 62mph because they tend to under read. if the car says 60mph its generally more like 58mph

I use my iPhone quite often with apps like runkeeper when I take the dogs for a walk to see how far i've covered, it overlays the path taken onto a map so you can see how very accurate it can be, but there are also times where it jumps about a bit, being a bit of a geek I also use it on the train to see how fast the train is going.
 
Bear in mind that the satnav is measuring your speed across the earth's surface and hills may cause inaccuracies, however, a good one will be more consistently accurate than the vehicle speedo.

Since all cars have ABS from 2004, most no longer bother with the gearbox output. The signals from the wheel speed sensors for the ABS are averaged and this message sent to the speedo, sometimes via the ECU. The message to the speedo may be a pulse, or a voltage. Pulse signals are interpreted by the speedo and a stepper motor moves the needle. Some are just voltmeters in disguise, so the needle just effectively displays the voltage supplied from the ECU, but the face reads in mph.

Each model can be had with various wheel and tyre sizes, all with slightly different rolling diameters. This used to mean different Gears in the gearbox to drive the speedo, so a change in wheel size meant changing the drive gear. (bit of a pain) Most manufacturer's these days choose the wheel/tyre options to give rolling diameters very close to each other. This allows them to set the speedo for the one with the largest rolling diameter, which would give the lowest speed reading and the one with the smallest rolling diameter, with a well worn tyre, should still be within 10%, so one size fits all. This means that if you've got the model with the largest option (not necessary the largest diameter wheel), the speedo is usually very accurate.

Does that all make sense?
 
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