Safe Speed Forums

Hi All,
Recently purchased Sat-Nav. Found out my speedo under read by 5 mph at 70 mph. I.E. if speedo is at 75 mph actual speed is 70 mph. Has been very handy knowing I have 5 mph in hand.

Partners car even worse it is reading 8 mph under at 70 mph.

Spoke to friend new corsa this reads 5 mph under at 70 mph.

He said older cars he has tried have speedo's spot on ?

This raised the question of governments dodgy data - if you choose to believe it. They say speed is of vital importance - why isn't the speedo calibrated accurately at the MOT.

If you have ever been stuck behind a car doing in a It may well be down to a new speedo ?

Richard

Not even checked. I had an escort a few years back. Passed its MOT three years running, and the speedo didn't even work.

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Speed camera policy Kills

I noticed this as well when I bought my sat-nav, speedo 70mph sat-nav 63-64mph.

May not be as bad as that though, I dont think the GPS system is 100% accurate, its optimised for America and can be out by up to 10 metres, this will obviously make a difference to accuracy of speed calculations.

The new European system Galileo promises accuracy to within 1 metre.

http://www.guardian.co.uk/science/story ... 35,00.html

Although there was an old thread about GPS speed accuracy (right down to ridiculous details - a bump in the road, a 0.000001% incline ), on a straight section you can be fairly certain that it will average out the time v distance. I too have my speedo saying 70, the GPS - 66, or GPS 70, speedo -75. I don't want to 'test it' past any scameras or talivans though

As for accuracy, it's down to the US government. They introduce fixed error algorithms into the data. Civvies normally get 5 to 10m accuracy at best. Even my old handheld GPS unit issued to UK forces had this level. The ONLY non-US military devices I've heard of having better resolution (30cm) are for geological survey. My receiver can pick up (and get fixes) at best on 11 or 12 sats. But I have to be in open space, clear skies, reciever external, pointing straight up, in general I get 6 or 7 good signals which is good for 5m (parked) to 10m (moving) accuracy.


ps. Does anybody else think it's a governmental conspiracy to have us driving cars with speedos that ALL seem to read under by roughly the same amount? How many of us have GPS 70, Speedo 65/66?

speedos aren't allowed to read slow.... hence usually they read high by a %

Not to be antagonistic, but "says who!". Where is the legislative paperwork / data to back up this?

If it IS true, then EVERY piece of GPS equipment ever sold is an innacurate piece of junk. I've used three different Sat Nav systems - and they ALL say I'm going slower than what my speedo says.

That's a TomTom 4 & 5 on PDA wired, TomTom Mobile 5 on Phone bluetooth, Garmin StreetPilot III wired, and oh yeh my old Ex-Army Trimble. As well as all the other reports in this thread.

The definative proof is of course time v distance. I prefer to believe my GPS over the cars electro-mechanical systems.

I realise that GPS works point to point, not on curves or gradients. But on a relatively flat, straight road GPS error can't be THAT bad.

exactly.... at typical driving speeds.. GPS will be more accurate than the speedo.

The regs are here - scroll down to para 19.

Basically a car speedo can overread by 0-10% + 6.25mph.
Which would be right if your speedo was overreading
Yep - me too

75/443/EEC



dont recall what the relationship stated is.... think its just a lower % threshold.

beat me to it BM..... uk implementation of the euro directive

Hi all,
Thanks for all the info. I find the 2001 regs incredible ! For instance at 70 mph. The actual speed can be 70/10 + 6.25 mph lower than the indicated speed. ( its not allowed to be higher )

Is this correct I.E. read 70 mph actual can be 56.75 mph

And at a reading of 30 mph actual can be 20.75.

This could explain a lot of the cars in the limits

and how dangerous is 56 mph on a motorway full of lorries ?

Paul how does this work with the 85 percentile surely this creates more danger ?

Richard

Ah! THAT'S what is meant be 'over reading'!! Man, do I feel stupid right now!!!

Found out that my speedometer was reading about 10% high when I carried out some tests with an LTI 20.20. If I had any doubts about the speeding alegation before, 47mph in 40mph limit with an LTI 20.20, this convinced me I couldn’t have been at the alleged speed. Asked the Police where I could get the speedometer calibrated, which just seemed to remind them that it was about time they took me to court, but I am still waiting for a date.

We should be grateful for the inaccuracies of GPS, since when the new European satellite navigations system is up and running, and is accurate to 1m, then we will all have to have satellite trackers in our cars for road pricing and big brother will then also be able to keep track of our location and speed all the time. The George Orwell prophesy of a police state will then come true, but a few years later than he expected.

Does anyone know of a satellite navigation system that can keep a log of speed and location against time for up to about a month, so that is the event of getting an NIP it would then possible to download the data to a computer and check the validity of the speed allegation. Even if this couldn’t be used as evidence in court, it would tell you whether it would be worth fighting the case.

Hi all,
I think I had a dose of enthusiasm and not enough intelligence. I made a cock up on the calculation. The speed V should be the actual speed. ( I used it as the indicated speed )

So an actual speed of 58 mph COULD indicate 70.05 on the speedo

i.e. 58 + (58/10 + 6.25)

i'm sure theres probably a commercial tracker/logger available for fleet markets by now.

but any handheld GPS will log it, so long as you have enough memory.. and if not you can always download to a pc at the end of each week.

dunno how submissable the evidence would be..... it wouldnt be exactly difficult to falsify a GPS log.

Well done Beermatt, you’re a star
I will soon broadcast this (and the implications of) to all and sundry.

Has anyone noticed how remarkably close the tolerance limit is to V/10 + 10kph ?

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Hi I think I have sussed out whats going on.

It seems that most speedo's are under reading 5 - 6 mph at 70 mph

Looking at the tolerance allowed i.e indicated 70 actual could be 58 mph to 70 mph.

In production design speedo to hit the middle of the tolerance band which is 64 mph. I.E. indicates 70 actual 64.

This seems to fit in with what most people with GPS speedo find.

I still have to pinch myself - my speedo is allowed to be 12 mph out. And the MOT 'safety' test does not test it !

It should be +/- 1 mph for the full range IMO. and checked yearly.

Richard

But only if you believe that the number it displays has any real importance.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

It can't ever be +/- 1 mph as tyre wear within the allowable wear limits can have a much greater effect than 1 mph. In fact tyre wear can increase the speedo reading by up to about 2% at a given speed, or more on winter tread tyres, and that's without considering the tyre pressure effect which can also alter the gearing by a couple of percent the other way. In Summer if you pump up your tyres on a cold day without having driven a long distance, then you drive on, say, a motorway or a twisty road, the heating up of the tyres can increase the tp's by a few psi hence increasing the rolling radius and hence the gearing and speedo reading becomes less accurate.

Assuming that road wheel RPM is the input of choice...

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety