Speedometer Error

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Neil Wyatt on 16/08/2015 14:16:07:

Interesting responses! My old car used to pretty much agree with the GPS give or take a couple of mph, but my Ford now reads 4 or 5 mph high.

I thought that a group of folks who expect sub-thou accuracy in their machinery might want similar results from their car speedos!

Someone mentioned type size and one or two tyre pressure – clearly plenty of room for variation there, what I twiggged the other day is the difference in tyre diameter (and therefore speedometer reading) with tyre wear between 8mm new tyre and 1.6mm (legal limit), total 12.8mm or about 2%.

Clearly no speedo based on tyre rpm can be guaranteed better than a few %, but it's interesting to realise just how much variation tyres contribute.

Hi Neil, my little Corsa reads about 1 1/2 MPH over true speed according to my Sat-Nav and my Astra is around 2 1/2 MPH over true speed. The worst car I had was my last Cavalier which was reading 10 MPH over true speed, always wondered why I never triggered a speed camera when I accidently passed them at what I thought was 78-80. My younger brother had a Cavalier which was reading about 5 MPH under true speed and for a long time didn't know it until one of his workmates was following him once.

I've always been surprised that the accuracy of the speedometer isn't included in the MOT.

Tyre wear has no significant effect on the reading of the speedometer, I once had both of my driving ones right on the legal limit and before I had them changed I noted the difference between my Sat-Nav and speedometer on a few different occasions and roads and the difference was always the same, when I had new tyres put on both wheels at the same time, I checked the speedometer with my Sat-Nav again on different occasions and different roads and the difference between the two was exactly the same as with the worn tyres.

Regards Nick.

Edited By Nicholas Farr on 16/08/2015 19:22:37

[AjohnwParticipant @ajohnw51620]

I'm talking about artics Nick or Prime Movers – the posh name in the trade. The ones that have a trailer behind them and usually have 2 to 4 axles on the truck. I think the max single axle loading is now 7.5 tonnes. I'm not sure about the speed the limiters are set at now but it was 56 mph / 90kph. The trucks are often fundamentally capable of going faster than that. I have heard comments about the limit being 50mph 80kph in the past but don't think that is correct.

I did find a link that goes through this area. There are probably others

**LINK**

As I haven't noticed any change in artic speeds while driving around I'd guess what it states on that is still correct. One of the things I used to ensure when towing a caravan was that I was at or above this speed. Anybody who has towed one will know why. As an aside I spent rather a long time working on tractor and trailer ABS on this class of vehicle also ADR's. The company was briefly interested in getting into limiters when the forthcoming legislation was announced. That is why they went into ABS as well.

John

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[Nicholas FarrParticipant @nicholasfarr14254]

Hi John, **LINK** HGV speed limit changes (blog)

Regards Nick.

[Ian HewsonParticipant @ianhewson99641]

my ceed gt shows 30 when the sat nav shows 27, but my mgb gt look 3 mph slow from my viewing angle whilst my wife says it reads spot on.

[AjohnwParticipant @ajohnw51620]

Posted by Nicholas Farr on 16/08/2015 20:09:41:

Hi John, **LINK** HGV speed limit changes (blog)

Regards Nick.

Yes I did see that and also read the comments below. Just to add to the confusion have a read of the speed limiter section of this

**LINK**

It's a .gov.uk pdf

I still suspect if anyone could makes sense of it all artics = 56mph. Actually I think that is agreed EU wide.

Personally I am more bugged by the overtake must be <= 10sec, if true. Since the limiters were fitted they generally can't overtake in that short a time on any road where they could because the limiters are set so precisely and generally take a lot longer that that which in turn holds every one else up particularly on motorways.

John

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[Neil WyattModerator @neilwyatt]

… Including some who commission them: The set on the Cat & Fiddle road being a notorious case in point.

MichaelG.

Interesting comment:

"downhill descent and uphill ascent"

Guaranteed to appeal to bikers, that….

Neil

[Peter G. ShawParticipant @peterg-shaw75338]

John,

Fair enough. My local stretch of the M6 doesn't have them other than occasionally for road works.

But, as a caravanner, I am limited to 50mph on single carriageways or 60mph on dual cariageways/motorways and thus can't avoid getting mixed up with HGV's which now have the same speed limits in England.

Judging by the portable speed display signs I've seen around, my car does indeed read high at 30mph – perhaps 2 or 3 mph. By that I mean that my speedo shows 32 or 33 when the roadside device shows 29 or 30. I assume that this applies through out the range which means that as long as I do not exceed the posted limit as shown by my speedo, then I should be ok. To be honest, if I'm travelling at the posted limit as shown by my car, then I don't care what other drivers are doing – if they want to overtake, then good luck to them – it's their licence, not mine.

What really does annoy me though, is, eg on the A75 in Scotland where the HGV limit is 40mph, mine is 50mph, and I'm being tailgated by a HGV.

Regards,

Peter G. Shaw

[martin perman 1Participant @martinperman1]

Re HGV's, until he retired a friend worked for a quarry company as their mechanic and they bought several eight wheeler tipper wagons for delivery work and they were governed which the drivers didn't like but they soon learned that if they used the hand throttle in the cab used to operate the tipper when unloading it in turn would overide the governor thus allowing them to increase the road speed using the throttle

Martin P

[Neil WyattModerator @neilwyatt]

> Personally I am more bugged by the overtake must be <= 10sec, if true. Since the limiters were fitted they generally can't overtake in that short a time on any road where they could because the limiters are set so precisely and generally take a lot longer that that which in turn holds every one else up particularly on motorways.

Eddie Stobart drivers are told to slow down if a plonker with a limiter set to 56.1 mph tries to overtake them. Not entirely sure they all do though

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 16/08/2015 21:07:34

Interesting comment:

"downhill descent and uphill ascent"

.

…and it comes from the very people who are "trying to get the message across"

For a slightly more detailed description of their ineptitude, have a look at this from 2011.

MichaelG.

.

P.S. … Yes, it's a dangerous stretch of road; and yes, there are a few too many wannabe TT Racers playing on it: But making it a one-way-street might have been more cost-effective than the cameras

[Enough!Participant @enough]

Posted by Neil Wyatt on 16/08/2015 11:55:34:

A bit of Sunday fun.
What's an acceptable error in a car speedometer?
There's a twist.

I reckon you could probably live with ± ten thou on the body of the speedometer with maybe another ± ten thou on the hole it fits into.

[Cyril BonnettParticipant @cyrilbonnett24790]

I can assure you that HGV's can travel rather faster than any posted speed limit in the UK, 90mph on an A class road approaching a bend! give or take 5mph.

[AlaninOzParticipant @alaninoz]

A few years ago I was going east across the Nullarbor. I saw a couple of trucks approaching in my RV mirror, I was doing about 107 in a 110 zone, contacted them on the radio and told them the road ahead was clear for about 5 km. They passed me, the second truck was about 1 metre behind the first, slipstreaming. I speeded up and tucked in behind, about 15 metres back and when I checked the speedo, it was showing 155 ! I followed them for about 50km until the road started to get a few curves. Incidentally, the world's second longest stretch of straight road, 146.6 km / 90 miles. The only variation is about +/- 10 metres in height. Driving at night you have to dip headlights about 20 km before you pass as the truck drivers, being so much higher are dazzled by your high beams even though you cannot see theirs. I may go east again in February to see my cousin in Melbourne.

[AjohnwParticipant @ajohnw51620]

Posted by Cyril Bonnett on 17/08/2015 00:08:19:

I can assure you that HGV's can travel rather faster than any posted speed limit in the UK, 90mph on an A class road approaching a bend! give or take 5mph.

I think there may be some confusion here caused by the use of the term HGV. There is one class where the trailer can't exceed the weight of the vehicle towing it that is an HGV. The classes are explained well on gov sites that go through the various driving licence codes. These don't seem to cover the case where the trailer weight does exceed the weight of the vehicle towing it. These are the ones that I suspect will still be limited to 56mph but MAY be changing to 60. What is effectively happening is that limiters are slowly being applied to lower weight range vehicles against a lot of resistance from people who use them.

Anyway I'm about to drive around 200 miles and as I worked on them always notice ARTIC's. I'll be interested to see if any are travelling at over 56mph.

John

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[Ady1Participant @ady1]

The a77 road to Stranraer had average speed cameras for a couple of years when I used it a lot, the speed being 50. Setting my cruise control at 55 and overtaking the 40-50mph muppets at 60 to get them behind me never elicited a response from the clergy

They are really only after the idiots, judges don't like their courts being cluttered with frivolous 2mph nonsense that serves no practical purpose for society

[Neil WyattModerator @neilwyatt]

They are (confusingly) called LGVs now.

HGV Class 1 /LGV C+E is for trailers heavier than the tractor i.e. 'proper artics'.

Speed limit for them is 60 on dual carriageway or motorway BUT new ones must be fitted with a 56mph limiter.

Amazing how many people don't realise that non-car derived vans are limited to 60 on dual carriageways – a fair few points doled out that way.

Neil

[AjohnwParticipant @ajohnw51620]

I did manage to sort the 60mph on real hgv's. Seems the uk did something on there own. The EU 56 was backdated and not just new as far as I am aware but past some point they are not needed.

Did you know that an empty Tesco artic clocks in at circa 17 tonnes and a hell of a lot of that is down to the racking. Actually I think it's 19.

John

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[John Stevenson 1Participant @johnstevenson1]

Posted by Neil Wyatt on 17/08/2015 10:29:06:

Amazing how many people don't realise that non-car derived vans are limited to 60 on dual carriageways – a fair few points doled out that way.

Neil

Sorry ALL vans whether car derived or not.

Friend was recently done in a Doblo van travelling at 70 on the A50 but a Doblo car, ie with side windows fitted is OK up to 70.

Also plenty of ways to bypass limiters on HGV's [ don't care what they call them this week, still HGV's in my book ] however the tacho will record that speed and not only the tacho which is vehicle specific it will also record that speed on the drivers card.

The drivers card is a card, same size as a credit card that is slots into the vehicle somewhere and linked to the tacho.

This records everything the driver does as regards hours, speed, time driving etc. The newer cards also have an RFI strip in them so Plod doesn't even have to pull you over, they can read the card as they drive alongside.

[MuzzerParticipant @muzzer]

When you drive in those 50mph average speed zones, it's clear that many drivers somehow believe that the speed thresholds for them is somehow lower than it would be for a static camera – like 47-48mph from what I can tell. Why on earth would that be?

As discussed above, the prevailing threshold allowing for the accuracy of the speedo is at least +10% +2mph, so you should be safe driving at 57 or so – assuming the driving conditions otherwise permit. Sure enough, I've driven all over the country on that basis and have still to cop a ticket. Yes, I have about 35 years of accident free driving with only one minor traffic offence (speeding, 15 years ago) in that entire period.

There's a lot of bull sitting when it comes to driving but also a lot of irrational thinking.

Murray

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Muzzer on 17/08/2015 14:54:33:

When you drive in those 50mph average speed zones, it's clear that many drivers somehow believe that the speed thresholds for them is somehow lower than it would be for a static camera – like 47-48mph from what I can tell. Why on earth would that be?

Perhaps they were screaming along at 55 before they realised they were in a 50 zone and have to bring the average down.

Rod

[Neil WyattModerator @neilwyatt]

Posted by John Stevenson on 17/08/2015 10:45:32:

Posted by Neil Wyatt on 17/08/2015 10:29:06:

Amazing how many people don't realise that non-car derived vans are limited to 60 on dual carriageways – a fair few points doled out that way.

Neil

Sorry ALL vans whether car derived or not.

http://www.gov.uk/speed-limits

The Doblo (I think) counts as a small panel van as it came before the people carrier,

Neil

[Gordon WParticipant @gordonw]

If you want to drive above the speed limit, based on the 10 p/cent etc. you have to be sure that your car speedo is accurate.

[V8EngParticipant @v8eng]

A story in the papers a couple of weeks ago, claimed that the Police are intending to clamp down on those exceeding speed limits by even small amounts.

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Roderick Jenkins on 17/08/2015 15:11:38:

Posted by Muzzer on 17/08/2015 14:54:33:

When you drive in those 50mph average speed zones, it's clear that many drivers somehow believe that the speed thresholds for them is somehow lower than it would be for a static camera – like 47-48mph from what I can tell. Why on earth would that be?

Perhaps they were screaming along at 55 before they realised they were in a 50 zone and have to bring the average down.

Rod

Hi Rod, I have done exactly that a couple of times.

Regards Nick.

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