I learned something new the other day.

My SATNAV shows a consistantly slower speed than my speedo - in any car. Its out by about 3-7 mph.

I blamed the SATNAV

Turns out that the SATNAV is as accurate as +/- 0.1mph on the flat.

Apparently all UK speedos are set to misread downwards of up to 10%, with most being 10% (especially accurate digital sender one's that don't use a gear-cam)

So when you're speedo says 77 you are most likely doing 70mph (and thus at 70 you're only doing about 64).

Another good use for a SATNAV and getting somewhere quicker!

Steve,

the letter of the law is that for speeds above 25mph, the error in your speedo reading shall not exceed (6.25 + V/10)mph where V is your true speed. The metric equivalent is (10+V/10)km/h.

here is a thread that covers this:

http://www.pocketgps.co.uk/modules.php?name=Forums&file=viewtopic&t=13472

Even you will be happy with this, martin - one of the posts says km/h (he's from Germany)

Steve, you can see the letter of the law at http://www.hmso.gov.uk/si/si2001/20010025.htm

As regards the errors etc associated with SATNAV readings may I remind you that I did a course on Kalman filters while I was at Uni and some years ago I was I was onnce invovled in a small project using Kalman filters associated guidance systems. SATNAV is merely another application of this technique.

You appear to be reminding me of something you did that I would have no prior knowledge of. How strange!

Anyways - is it not possible that the "rule book" you have produced pretty much concurs with what you read on the posting I gave you.

I assume you did ACTUALLY READ the link?

Know it does not endear me to you Steve,
In fact it makes me wish to your head cleave;
And I say all this because I love you,
And I should know that rage be above you,
You, that fair and full-formed hunk of man
Who's sent metronics back to garbage can.
Why have I burst into sudden rhyme so pall?
I cannot say, but God bless each and all.

Bryan,

Give me that bottle

[struggle struggle]

[SMASH!]

*NOW* Look what you've made me do!!

heh, maybe a little bit I've had.

Martin,
The law you quote above seems extremely suspicious to me. If the speed limit is 30 mph, for example, you could be legally driving at almost 10 mph faster, blame it on your speedometer, and get away with it. That is 33% over the limit, which seems quite ridiculous.
I don't mean to question your information. I'm only making a comment. Maybe exceeding the speed limit legally is meant to be very easy in Britain.

Bud,

The URL that I gave was

www.hmso.gov.uk/si/si2001/20010025.htm

Anything with .gov.uk is from a UK Government Department. I do not believe that it is possible to create a false address containing that extension. The fragment "hmso" stand for "Her Majsety's Stationery Office" and is the place where copies of any Government document can be bought.

Having told you that, do you still think that the law is suspicious? As far as I am concerned, that address is absolutely correct.

If I did 40 on a 30-road past a school at kick out time and there was a police patrol there I'd like to wager 60 quid and 3-points that Mr Constable will be flashing at my rear view mirror before I can say "please look at the link I posted above for people discussing this very subject from a GPS/Speedo perspective....please"

But, if instead of sending you driving licence and a cheque for £60 to the local prosecutor, you sent him a letter quoting chapter and verse of the Statutory Instrument and telling him that if the case was not dropped you would be handing it over to your solicitor, he would probably drop the case.

Er, the Road Traffic Act is absolute in regards to 'speeding', i.e. ignorance is no defence. If the law says X, the limit is bang on X and not 1 km/h faster (I thought I'd get that one in!).

The only reason that the plod allow a little leeway is that under criminal law, an offence must be proved 'beyond all reasonable doubt'.

Martin, like I said earlier, I don't mean to question your information.

But I somehow have a hard time believing that you can be driving 33% faster than the posted speed limit and get away with it on the grounds that your speedometer was inaccurate.

AndyA - Have you actually read the rest of this thread? I suggest you do!

<<But, if instead of sending you driving licence and a cheque for £60 to the local prosecutor, you sent him a letter quoting chapter and verse of the Statutory Instrument and telling him that if the case was not dropped you would be handing it over to your solicitor, he would probably drop the case.>>

Yeah- I reckon that'll really work! <-- Sarcasm



When I do 30 mph in my car the SATNAV says 27. being electronic and calibrated to fit in with law I would suggest that 33 is the fastest you could go in a 30 zone before saying good-bye to 3-points on an SP30 charge (or XP30)

Bud - Please take a look here:

http://www.pocketgps.co.uk/modules.php?name=Forums&file=viewtopic&t=13472

for people discussing calibrations.

If you were to read the text of the page on the HMSO site at
www.hmso.gov.uk/si/si2001/20010025.htm as quoted by Martin earlier, you
would all save yourselves a lot of wasted time. Look carefully at point 2
below as reproduced from tha HMSO site. This says that a speedometer may be
in error by the specified amount ONLY if the indicated speed is LESS THAN
the true speed. So forget about doing 39.25 mph in a 30 mph limit and
claiming your speedometer was in error but within allowed limits and only
showed 30. If the speedo indicates less than the true speed and the true
speed is over the limit, you're done..

Speed-ometers
1. The vehicle shall be fitted with a speedometer capable of indicating
speed in mph at uniform intervals not exceeding 20 mph at all
speeds up to the maximum speed of the vehicle and capable of being read by
the driver at all times of the day or night.

2. For all true speeds up to the design speed of the vehicle, the true
speed shall not exceed the indicated speed.

3. For all true speeds of between 25 mph and 70 mph (or the maximum
speed if lower), the difference between the indicated speed and the
true speed shall not exceed -

V/10 + 6.25 mph

where V = the true speed of the vehicle in mph.

OK, that's much more believable.