Safe Speed Forums

Despite the fact that I have come in for a lot of flak on this site (a lot of it I probably deserved!) it has made me think about my driving. Which is probably a good thing. Athough there is an argument that it's taking my attention away from the road ... lets not open that one here!

Anyway, as I was driving home tonight I was practising looking at my speedo without refocussing, and I noticed something that I had previously not noticed (at least consciously).

On my car, a Peugeot 206Gti, the speedo is calibrated so that at an indicated 70mph the needle is exactly vertical, and at 30mph the needle is exactly horizontal. As probably 95% of my driving is on motorway (certainly 95% by miles covered if not by time spent) this means that my main driving time is spent in a state where all I need to do to see if my speed is at or below the posted speed limited is to make sure the needle points upward. Similarly at 30mph the needle needs to be flat.

It means that each mph is slightly more than 1 degree, and it's commonly known that we can detect an angular variance of much less than 1 degree (you know when a picture is hung ever so slightly off straight - most of us can detect a variance of a fraction of a degree although sometimes we don't know why the picture just doesn't 'look' right) although this works best at variance from horizontal or vertical.

This seems to be a really good design, I wonder if it is accidental or if it was a conscious design decision? Obviously this falls flat at other speed limits (40 / 50 through the roadworks from hell at Heathrow).

That seems to be true of most cars made for the British market.
On my (german) car 70mph is at around the 10 o'clock position.
I wonder what would happen if they changed the motorway speed limit? Would new British-market cars have redesigned speedos to reflect this?

Cheers
Peter

doesn't mean your speedo is right though. Do speedos get different calibrations for different tyres used across a model range? Or different tyre makes and model, or when Darren and Kevin put noisey wheels on their souped up nova?

I have to change the calibration number on my tractor when I change wheels for the speedo to be accurate. But when I last changed them I checked the rolling radius against the figure for that tyre size in the book and it was 200mm different. Forward speed is critical in my line of work, but I get fined if I get in wrong in the car, oddly...

I'm not a darren or a kevin! Thankyou very much. I also wonder if D or K bother to tell their insurance companies about their mods so that they can be sure of cover?

I've checked my speedo agaist the GPS (0.11mph accuracy) and when the speedo reads dead on 70 I am doing 66mph on the GPS. That gives me a 4mph tolerance before breaking the speed limit.

As I said, though, it's not perfect as it only works at 70 and 30 and these aren't the speed limits everywhere.

I wonder how european drivers cope. British cars have MPH and KM/h on the speedo but some Foreign cars have only Km/h

I wonder how they do the "instant" conversion when they drive their cars in this country...

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Speed camera policy Kills

If I were driving in a country where the limits were signed using a different set of units to those marked on my speedo, I guess my response would be either:

1. print up a small aide memoire card showing the equivalent speeds for each limit, and refer to it before each journey until I'd memorised its contents.

2. realise that the local scammers are just in it for the money, won't pursue a foreign-plated car because it'll cost them too much, and drive around without being too worried whether I'm a couple of mph/kmh/whatever-per-whatever over the limit and at risk of a camera activation...

Handy wrote


Thats about 6% out, I thought speedos were more accurate than that. So when I'm doing 77mph on the motorway it could be over 80.

It's (roughly) easy. 30 mph is 50 kph, 40 mph is 65 kph, 50 mph is 80 kpm, 60 mph is a tadge under 100 kph, and 70 mph is 110 kph. If a person is really bad at arithmetic, they can cut up a sticker, write on the numbers and fix them on thier speedo glass at the right place.

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I stole this .sig

Speedo can indicate 0 - 10% over actual speed. They cannot be 100% accurate because of tyre wear, and normally vary across the speed range too. Speedo should never indicate a lower speed than actual.

My new GPS navigation system has an "Over the Speed Limit" warning which is bloody annoying as it has zero tollerance, but has confirmed that my speedo is dead-on at 30 mph. This confirms that the Police cars on the 30mph ring-road are regularly exceeding the speed limit, and that it isn't just due to them having calibrated a speedo.

It's actually 0-10% + 6.25mph (see regs here, scroll down to para 19).
So to answer Rewolf - when doing 77mph on the motorway, a legal speedo could read anywhere between 63.25mph and 77mph.

But what do they have to worry about?

What's going to happen if they trigger a camera? Not a lot - not really cost-effective is it?

Get stopped by a policeman? Become unable to speak English, and (unless you were really taking the mick) it's unlikely to be taken further..

NO, NO, NO.

OK I missed out the 6.25mph, but it is the opposite way round: when doing an INDICATED 77mph on the speedo the car can be doing an ACTUAL speed of between 63.75mph and 77mph. Section 19.2 explicitly says: "For all true speeds up to the design speed of the vehicle, the true speed shall not exceed the indicated speed".

If when doing an actual 77mph the speedo says 64mph then you will trigger every speed trap in the country, and at an indicated 70mph could actually be doing 83mph! A speedo must always be accurate or read a higher speed than you are actually doing.

This is the situation reflected in Handy's speedo where an indicated 70mph is an actual 66mph.

This wide variation in permisable speed limits also explains why some drivers think others are too fast or slow - they both think they are right because the speedo tells them so, while the driver with the accurate speedo thinks the other driver is a deliberate pain in the arse by doing 50 in a 60 - the driver with the over reading speedo thinks everybody else is a reckless speeding idiot and wonders why they cannot keep to the limit like they do all the time.

OOPS!!!

Many apologies - yes, you are correct, I wrote the complete opposite of what I meant (i.e. indicated 77mph can be anywhere between real 63.75mph (thanks for correction) and 77mph)..

Brain obviously not in gear at 3:30am..

I know my speedo overreads by about 7% at 70 mph but when I am being observed for my IAM training I have to forget this and stick to what the speedo says. So if I am holding you up it's not entirely my fault.

So at 20mph it could read anything at all? Are they prosecuting for 20+10%+2 (i.e. 24mph) in 20 zones?

This would seem to be covered by requirement 2 from the page beermatt linked to earlier:

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

Which still allows it to read 24.999999mph for all speeds up to 25mph. In fact I see nothing to disallow a reading of over 25mph until the vehicle actually reaches 25mph. But that would be silly.

True, though your earlier question asked if the speedo could read anything at all, which isn't quite true. I see what you're getting at with the 10%+2 enforcement limit in 20MPH zones, but maybe that's why such zones never seem to be rigidly enforced - the ones I've experienced so far don't have any cameras, and appear to rely on chicanes, humps etc. to self-enforce the lower limit.

Well no, it can't read under your actual speed but it can read anywhere between your true speed and the speedometer's maximum. If it were to leap to 160 as soon as you start the engine and stay there until you hit 25mph it would be within regulations.

Just does not seem fair to enforce a speed limit unless we are entitled to the tools with which to comply with said limit.

I am sure most speedometers are up to the task but it is typical of the disjointed thinking.

If you don't have GPS, a reasonably accurate way to check your speedo at 56mph is to follow a convoy of trucks on a LEVEL road. They won't be far out.