Speedometer accuracy

[Guest]

Like many of you, I'm sure, I have finally succumbed to the need for a speed camera detector (road safety device!). For various reasons I have acquired an "Originb2", which seems to work very well in my Boxster. However one thing puzzles me, and I can't believe that I'm the first Boxster (or Porsche) owner to discover this. I have discovered a large discrepancy in the recorded speed between the "Originb2" and my car's digital display. The Porsche OVER-READS by about 10%, apparently at all speeds. I assume that the "Originb2" being GPS based is accurate. Do Porsche speedos typically over-read by 10%, or is mine just wrong? Is their a computer adjustment for wheel/tyre size?
Any guidance would be much appreciated.

 

[daro911]

ORIGINAL: David Bricknell

I have discovered a large discrepancy in the recorded speed between the "Originb2" and my car's digital display. The Porsche OVER-READS by about 10%, apparently at all speeds.

I believe by law a cars speedo's must over read, as opposed to under read, but don't quote me and by how much would probably be a whole different can of worms []

As an aside my OBC over reads the mpg by around 10% too so maybe there is a conspiracy theory kicking in here

 

[Guest]

I have checked my Boxster with my GPS unit. The car speedo overreads by 10% at 30 mph ie 3mph but only by 5-6mph at 70mph and remains at about this at 80-90mph.
Remember that there is a slight delay in the GPS ( road safety device) making a recording since it is recording two positions on the earth and calculating speed from the time interval. Check speeds shown against the car speedo as the car decelerates compared with those accelerating. Claerly if your spped is stable then this error does not occur.

 

[Guest]

The speedo must not under read. This would allow you to travel faster than you think you are which is a safety issue.
The accuracy of the speedo depends on how well it was callibrated (although this should not affect the digital speedo), manufacturing tolerances and how worn the tyres are. As the tyres wear the rolling radius and hence circumference gets smaller, so for a given number of revolutions of the output shaft on the gearbox, you go less distance.

New tyres have 8mm of tread I think, at when at the limit have 1.6 mm, so this means the circumference is reduced by about 2%.
You will also find it gets less accurate if the tyres are under inflated.

There is a limit to how much it is allowed to under read, which I thought was 10%.

I would expect the computer to take the fuel consumption and compare it to how far it thinks the car has travelled, based on the speedo.
The sat nav can use this as well when it is out of coverage, e.g. in a tunnel or under trees. I don't know if the OBC can check itself against the sat nav.

I think that 10% out seems a little high though, I would expect it to be no more than 5%.

BTW each revolution of the tyre covers just under 2m.

 

[Melv]

ten percent sems to be the norm for most vehicles.

Which means if you drive at the limit + 10% + 2mph (Police guidelines), you won't get nicked!!

Mel

 

[JCB..]

My Sat Nav consistently reports that I am doing 4MPH less than indicated.

I have very recently put new rubber on and it still indicates the same difference.

JCB..

(I use my eyes for detecting cameras - much more reliable)[]

 

[thboxster]

I noticed the same thing when I got my GPS installed. Doing a constant speed, so as not to confuse things with accel. and decel. 50mph on the GPS reads 54 on the dash and the dial is more or less 54 taking into account the thickness of needle and the close ratio of the numbers.

So, when I see a speed camera, as long as my dial says no more than the speed limit of the road, I know i'm safe.

As an asside:

I did manage to get to about 40mph before the GPS realised I was moving. []

Something else to mess about with.

 

[TopBear]

My speedo reads 30mph when the B2 is saying 27mph.

At speedo 80mph the B2 says 77 as well...

I never go above 80, so cant comment about higher speeds[]

 

[Guest]

Hi

I have road Angel and 10% is what I got

seems the norm,

I wonder what the traffic cars would record for average speed? Do they record via GPS or via their speedo? does anyone know?

 

[Mark_Bennett]

They have large caliberated speedo's (or at least, they used to have).

 

[Guest]

Dear All,
Many thanks for your prompt and useful replies. I've put new rear tyres on to-day (Michelins) and have tried to do a retest.
The error is certainly 10% at 30mph, but does seem to get less at higher speeds. Like Peter Hollins the error seems to be about 6-7 mph at +/-85.....I haven't had the opportunity to go faster yet! Maybe the greater circumference of the new tyres has had some effect. No wonder I always seemed to attract queues behind me in built-up areas!