27 replies to this topic

[AntsClubby]

Ive had an idea..ive thought of installing a satnav into my dash...i then thought could i use the gps inside of it as my speedo...itl be far more accurate than a mech one in my opinion!

In the UK do i need to have a mechanical speedo fitted by law or can i just use a gps one?
I cant find any "certain" info on this, maybe im searching the wrong terms?
Does anyone...ideally a copper know a definate answer?

Cheers
Ant

[dklawson]

Sorry, I cannot answer the legal questions but I do have an acquaintance who recently fitted an honest-to-God GPS speedo in his Mini. It's not a Satnav system, it's an actual analog speedometer that determines speed and distance using GPS signals.

Take a look at this link...
http://www.speedhut....dometer_GPS.htm

[Spitz]

Wow....I like those ( I think I'll fit the 260mph one...lol )

Obviously you need to purchace/mount the antenna also.
So if you go in a tunnel or similar.....your speedo would quit working I imagine.

[Dan]

Why do you assume a GPS speedo will be more accurate than a well calibrated mechanical one? Domestic or civilian GPS units are intentionally vague.

[AntsClubby]

I dont want to put my old speedo back in as it looks really dated now, a new speedo is around £100-£150 so i just thought that a satnav style speedo could be an idea.
I was always under the impression that a gps was more accurate than a cable fed/std speedo?
As long as its within 2-3mph ill be happy (speed cameras etc)
Just need to know if its legal...could always fit a dummy one incase mr copper stops me ;o)

[Doz1971]

Why do you assume a GPS speedo will be more accurate than a well calibrated mechanical one? Domestic or civilian GPS units are intentionally vague.

Domestic/ Civvy GPS units have the POTENTIAL to be vague, but this is controlled by the US military (it's their network). However, currently, GPS is extremely accurate. THE US Military have the ability to reduce system accuracy.

[AntsClubby]

What Does 'very' accurate mean?...within 1/2mph, 10ish meters?
Is it the kind of thing they change often then?
Prob explains why my satnav has fits most trips i make!ha!

[Guess-Works.com]

The speed indicated by a satnav is a mathematical product of the distance traveled over time therefore it likely to be more accurate than the actual position as it's likely any errors on position would cancel themselves out over a small interval.

I personally would trust a satnav given speed over an uncalibrated mechanical one.

[AntsClubby]

So does anyone actually know if its legal to have a car on the road with just a gps speedo?
Think i might just ring the police and ask them??

[reallybig]

satnav and gps are two different things

if your getting one for speed alone you need one specifically designed just for speed

the satnav ones are only vague esimates and gps signals are crap in mountainious area and built up cities

[Ivor Badger]

The legal requirements for a speedo are.
1 Must be accurate to within 10%
2 Must be visible to the driver
3 Must be illuminated at night.

As most satnavs or Decca navigators in their day worked in giving you the speed over a previous set time. Ie, took the position change in the time period and then worked out the average speed. 4 mins in Deccas case. this once resulted in an average boat speed of 75kns, which was quite inpressive as we were drifting while waiting for the signal to correct.

[lrostoke]

http://www.pistonhea...ic.asp?t=957174

may be worth a read, I think basically, it'll pass an MOT because speedo isn't tested, but its C&U laws it may fall down on..
Although in that list on the link above nothing about it needing to be connected to the car.

The accuracy only seems to apply one way, eg the speedo can read high, but can't read low.

So basically you can't use the speedo as a defence for speeding because it should either read correct speed or one higher than you are going

Edited by lrostoke, 10 April 2012 - 11:32 AM.

[Doz1971]

Urban canions are an issue. MOST (but not all) GPS units pull their speed directly from the GPS chipset involved. Ublox, Navistar, and SirfStar chipsets provide this data for the external product. The update speed on, say a Garmin or Tom Tom is delt with by the device, but the data shoould be very accurate indeed. Most of the units I deal with, which are fitted with UBlox, are able to determine which side of the road they are on, let alone which road. Acurracy is better than 3 metres. It does depend how many satellites the unit can see.

[Doz1971]

The legal requirements for a speedo are.
1 Must be accurate to within 10%
2 Must be visible to the driver
3 Must be illuminated at night.

As most satnavs or Decca navigators in their day worked in giving you the speed over a previous set time. Ie, took the position change in the time period and then worked out the average speed. 4 mins in Deccas case. this once resulted in an average boat speed of 75kns, which was quite inpressive as we were drifting while waiting for the signal to correct.

Decca navigators ... LORAN... now those were the days

[AntsClubby]

Well ive spoken to the police, they said i need to have some kind of mechanical or electric operated speedo incase my gps unit fails, satellites pack up etc.
I can use a digital one with cable sensor, as newer cars or one like spa gauges do but not a dedicated stand alone such as a drift box or tomtom type with no physical connection to the car.

Oh well, looks like its analogue still for me then...worth a shot though!
Thanks for all the replys though
Ant