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GPS Speedometers


rtbarton
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I've just acquired a Volvo with a duff speedo and getting a replacement is a bit of a nightmare so I was thinking of using a GPS device instead. Ideally I'd want one I could move into my Landrover when that speedo cable breaks!

Do they indicate instantaneous speed or average speed over a distance?

Any input much appreciated!

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I've just acquired a Volvo with a duff speedo and getting a replacement is a bit of a nightmare so I was thinking of using a GPS device instead. Ideally I'd want one I could move into my Landrover when that speedo cable breaks!

Do they indicate instantaneous speed or average speed over a distance?

Any input much appreciated!

I don't use it as a speedo (cos mine works I guess), but my pda with CF gps gives speed and avg speed. Very useful to check what speed you are actually doing, rather than what the Defenders analogue reading with overgeared 285's says it is! :D

The readout is quite small and accompanied with lat & long etc so not clear in an 'at a glance kind' of way, but good enough to use as a safety check.

Hope that helps

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Hi.

In just about every car I have been in using a satnav, the car's speedo seems to show 3 or 4 mph more than the sat nav does.

I think the sat nav would be more accurate but only in a straight line. And they do seem pretty instantaneous. IMHO

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Because a speedo usually over clocks speed it could just save you at speed camera's etc

It could be an offence to not have a working 'normal' speedo. Wonder if you would get away with having a gps as a speedo.

Just thought I would throw that one in.

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Remember though, that gps speed is only accurate on the flat, hence it's suitability for ships, on hills the readings are out.

Mo

That's a good point, presumeably it will read slow. I don't suppose any incorporate an inclinometer :lol:

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I use an old Garmin GPS III with the 'large figures' option on, it's dead easy to read even for my old eyes which need reading glasses. It also shows the time accurately which is handy. And it can be backlit nicely at night. In fact I never switch the thing off any more as the current drain on the battery is very low. The only problem is it 'loses' the satellites when parked in the garage and sometimes has to be switched off and on before it reaquires the satellites.

Of course if I had a GPS as used in 'The da Vinci Code' (that works in the basement of the Louvre), I wouldn't have this problem :rolleyes:

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calibrate the rev counter? then if the GPS drops out you can still make a good guess at the prime speeds.

I did have a program that worked on PDA to show you the speed as a proper analogue speedo, but i lost the source code and have not got around to making a new one since. There are commercial versions around though, I am just to tight to buy something I can write myself.

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calibrate the rev counter? then if the GPS drops out you can still make a good guess at the prime speeds.

I've been looking for the mph/rpm figures but can't find any and there is such a range of diff & gearbox ratios that I wouldn't like to calculate it. It's made slightly more inconvenient by being automatic.

I've no intention of using those yellow boxes the government provides!

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GPS units give accurate speed measurements, but averaged and updated once a second. I challenge you to make a significant change to your speed in a second, in a situation when you need to know what speed you're doing.

GPS speed is a 3D vector, they work on hills, perfectly accurately.

I drive by my GPS speedo, far more accurate than the gearbox driven one. Though i guess there might be legalities on driving without a standard speedo, odometer.

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GPS units give accurate speed measurements, but averaged and updated once a second. I challenge you to make a significant change to your speed in a second, in a situation when you need to know what speed you're doing.

You haven't seen my driving :rolleyes:

GPS speed is a 3D vector, they work on hills, perfectly accurately.

I was having this conversation with a neighbour just now and he thought the same.

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They work accurately according to the reading they see for altitude, but the dilution of precision is quite large vertically, if only because the satellites in view are usually above you - sit still and watch your altitude drift around. Most use an algorithm to smooth this at low speeds, but this inaccuracy was a major downfall in my GPS rolling road programme.

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GPS needs three satellites to get a fix, most modern units are 12 channel, therefor capable of receiving data from plenty of the satellites up there i.e, the ones nearer the horizon. Yes the accuracy in altitude is diminished due to the geometry of the configuration, but average change in vertical speed is low, especially when considered as a factor of forward speed. given this ratio, and the perpendicular relationship between horizontal and vertical planes, I'd say the effect is largely negilgable, under normal circumstances (rolling road excluded:)).

Certainly within the scope of 'is GPS accurate?' the answer is yes, very. Ultimately the errors come down to the quality of the reciever. There was/is an argument for DGPS, but since the US turned SA off, you really can't go far wrong.

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The reason the altitude fix is generally less accurate is because land or sea based GPS wants to get the most accurate 2D fix it can. It does this by choosing sat's as close to the horizon as possible. Aviation GPS's tend to pick a few which ate closer to being vertically above giving better altitude at the expense of 2D accuracy.

Most GPS's and 3rd party software use the GPRMC string in the NMEA data string (because it contains almost everything useful). The speed included in this and most of the other strings is described as "Speed over ground" and is only calculated from the 2D data - giving you an artificially low reading on hills.

Recent GPS's will spit out so called interpreted strings - these are are readings which the gps has calculated from other raw data. There is one of these, GPRMV which includes true, 3D velocities.

There are a couple of other strings which include true velocities to a destination waypoint - but for continuous velocity measurement, these are difficult to use (because you have to set your destination to the last 3D fix you got, then the velocity on the next reading is accurate, but negative. you then reset your destination and repeat. the down side of this is it is difficult to get more than a single reading every 3-4 seconds.

Although in theory, yes your GPS can spit out a true 3D velocity - in general most, and most bits of software only spit out 2D - so watch out for speed cameras on hills! My Garmin 5 is spot on compared to my TerraTrip thing on the flat, but can be 5mph under on hills - enough to get the odd point!

How do I know all this? In 1995 I was part of a team building the very first GPS in-car navigation system which ran on a Psion 3c - and was very good. I actually still use it (ported to a Series 5). There are two bits of code still floating around on the web, one converts from Northing / Easting into Bartholomew NGR's (Eg. TQ123456) and the other goes from Lat/Long to Northing/Easting - which I wrote!

The only mention of it I could find on the web:

" FindIt from Widget Software Ltd. +44 143 881 5444

FindIt provides drivers real-time information on journeys. It

can also display details about road junctions, petrol stations,

fast food outlets, banks and 12 other databases"

Si

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Si,

Interesting, i'd not realised there were different NMEA strings for vertical accuracy and horizontal accuracy.

I tried writing some code to read NMEA strings a few years ago, using a serial terminal in VB, i gave up after a while and used the eTrex's serial word output, which was enough to get my home-built GPS map plotting software working. The project died when i got my hands on Memory Map.

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i use a talex lite gps which is ideal as it is about £50 from ebay. very simple and clear speed reading all the time (i find using a tomtom for speed reading is carp as it looses signal when under trees and is too small to read ect. it shows the time when you stop so looks like a clock to all those lite fingered window shoppers!

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Looks like I've got a working speedo now.

Thanks for all the comments, very interesting stuff!

The Volvo instrument pod would look nice behind the steering wheel of my SIII, and the v6 would look very very nice under the bonnet....................................................................

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Spot on! (almost) 1989 760 GLE Estate. The saga is documented here Volvo Speedo

My dad had one of thoses and the speado works in revers on them. Did 120mph in reverse. But it was on mud :D:D . And his was auto so you could put your foot on the brake and the other on the loud pedal and it would do a 100 while sat in the same spot (well with the back wheels spinning :):)

Well come on i was 18 :)

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