vRS Speedo Accuracy ?

[Roger M]

Hope this is the right Forum ! Any of you experts have definitive info on the speedo accuracy in general ?? To achieve - say - 70 mph on my Origin b2 I need 73 - 74 mph on the speedo. Ummmm.... which one's right, if either ?? Roger M

[galthop]

VW group instrument clusters add 3% to the speed that they think you are travelling at. Then there will be inaccuracies due to tolerances on all the mechanical components - is the pointer attached to the speedo at the correct angle, how worn are your tyres, etc. What you are seeing sounds about right to me.

[robmawer]

The GPS device - it measures how far the car has actually travelled, rather than trying to guess it from the wheel revolutions.

If you do a search on it, there are lots of people who've noticed the same thing.

Skoda UK insist that it's accurate though!

Rob.

[DGW]

GPS should be very accurate when speed is being measured while travelling in a straight line. This thread gives typical feedback from members on this topic.

[Roger M]

Thanx guys - my "flabber" is truly "gasted" at the speed of response. I was a little wary of the GPS speed, as we're not travelling in a truly straight line, and I don't know how often it samples the position to compare with the last one to derive speed over the ground (say if turning a bend) - but I guess it must be a high rate continuous process.

Again, WOW & thanks. Roger M

[Pythix]

I have been 'gunned' at 74ish on the speedo, 70 on my S6R.........and the reading was 70.2. Quite happy with the GPS accuracy, but have noticed the errors creep in on steeper than average hill climbs and descents.

I have used a flat, straight road and done 'calibration' runs and 5-6% seems a good rule of thumb to safely go over the posted limits below 100mph.......after that is a different story......so I'm told!!!!

[wilsonan]

I was a little wary of the GPS speed, as we're not travelling in a truly straight line, and I don't know how often it samples the position to compare with the last one to derive speed over the ground (say if turning a bend) - but I guess it must be a high rate continuous process.

Roger,

My Road Angel updates every second (as long as it has a clear view of at least 3 satellites to triangulate it's position) I presume the Origin B2 is similar?

Andy

[flyingscot]

If you want to check it without the need for a GPS system find a quiet stretch of M-way and use the 1 mile marker boards. Drive at 60 mph and time yourself, it should take longer than 60 seconds and from the difference you can work out your speedo accuracy.

[Roger M]

Thanks Andy - hadn't got as far as bothering Origin for fix rates, but they must be the same. The stopwatch idea doesn't appeal too much, 1 second in 60 is more than 1.5% !! Anyway, I'm happy now I know which to trust more........ Never even thought about in in me Airbus, radar's there to help us.............

Roger M

[spinndrift]

I presume the dodgy reading for hills would be something to do with a slight change in altitude.

[MonkeyHanger]

145 indicated is 140 GPS on mine

[Pythix]

I presume the dodgy reading for hills would be something to do with a slight change in altitude.

Once up the top or down the bottom of the hill, the accuracy returns. I think it's to do with the line of sight/angles thingy between satellite and GPS receiver.

[mannyo]

Thats right, If you are going up or down hill, to travel the same horizontal difference you need to go faster as you need to travel slightly further.

[spinndrift]

At the end of the day all the satellite gubbins was diveloped by the military for working out where you are and not how fast you're going.

No doubt someone will come up with something that will measure signal time from car to satellite and be able to adjust the speed reading for going up and down hills.

[MonkeyHanger]

At the end of the day all the satellite gubbins was diveloped by the military for working out where you are and not how fast you're going.

.

It's been proven plenty of times to be more than accurate for most peoples needs, < 1% IIRC.

I'll take something that checks my speed every 10 metres over a needle on a dial any day, thankyou

[andy ellis]

Most speedos over-read by about 10%, mine was 10% before I up sized the wheels and is now about 5%. The gps seems most accurate, chech also against those signs that show your speed

[Pythix]

The official 'allowance', so I'm informed, that a speedo may over read is 10%+6.25.........speedo says 70, could be doing as low as 57 and a bit!!!!!

[flyingscot]

The official 'allowance', so I'm informed, that a speedo may over read is 10%+6.25.........speedo says 70, could be doing as low as 57 and a bit!!!!!

I don't think it's as much as that! It is max 10%+2mph...

[Pythix]

I don't think it's as much as that! It is max 10%+2mph...

I think the 10%+2mph is the allowance before prosecution.

I'll have to find the link to the vehicle standards whatnot I found it in. Basically a speedo in a car can't under-read, only over. The formula used so that it doesn't get silly is 10%+6.25....When I find it tomorrow at work I'll post a link or something.

[GAFF]

I found with GPS that my speedo was indicating 70 at 63mph, when coming back from Donegal the other day there was a board at the side of the road which showed your speed, at 70 on my speedo it indicated 63mph like the GPS.

[MonkeyHanger]

10% +2 is the ACPO guideline before a speeding prosecution is launched.

It is ONLY a guideline and not a rule, so don't take it literally

[steve]

I found with GPS that my speedo was indicating 70 at 63mph, when coming back from Donegal the other day there was a board at the side of the road which showed your speed, at 70 on my speedo it indicated 63mph like the GPS.

My RS reads 80mph and 73 on the GPS.

Gaff i once got stopped{in Ireland } doing 97mph in my mondeo while visiting the mother-in-law, i'd only been off the boat 45 mins {in the middle of nowhere- the f***ing field started flashing with blue lights -freaked me right out}.The policeman or Gaurda ask where we were going in such a hurry.At the time i had some err unusual roll ups with me :eek: . he asked our destination which was a cul-de-sac in the middle of.......very nice he was... just told me to slow down or else .

3 hours later we arrive at the 'old trouts' about 2am and guess whats parked out side her house ....the ****ing old bill .As soon as we pulled up they drove away.Very slowly.

Weird.

Steve

[mellowyellow]

At the time i had some err unusual roll ups with me :eek: .

Steve

Obviously not Golden Virginia

[steve]

i had an oz but not of Golden V .

Steve

[Pythix]

This is what someone in the know sent me a while back when I approached the question of why the speedos can be so far out:

1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at all speeds up to the design 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' date=' the true speed shall not exceed the indicated speed.

3. For all true speeds of between 25 mph and 70 mph (or the design speed if lower), the difference between the indicated speed and the true speed shall not exceed - [b']V/10+6.25 mph where V= the true speed of the vehicle in mph[/b].

This item shall not apply to vehicles having a maximum design speed of less than 25 mph.