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Joined: Mar 18, 2011 Posts: 4462 Location: West Sussex
Posted: Mon Sep 17, 2018 4:46 pm Post subject:
MaFt wrote:
If the NIP says 71mph then they are saying that their speed readings are 98.6% accurate (1mph is 1.6% of the 70 speed limit so to prosecute at 71mph they are confirming it is accurate to that sort of speed difference).
Are they though? Maybe they've already allowed for the tolerance so let's say they allow 5mph, car was clocked at 76 so NIP says 71 _________________ Where there's a will ... there's a way.
Joined: Aug 31, 2005 Posts: 15356 Location: Bradford, West Yorkshire
Posted: Tue Sep 18, 2018 11:16 pm Post subject:
sussamb wrote:
MaFt wrote:
If the NIP says 71mph then they are saying that their speed readings are 98.6% accurate (1mph is 1.6% of the 70 speed limit so to prosecute at 71mph they are confirming it is accurate to that sort of speed difference).
Are they though? Maybe they've already allowed for the tolerance so let's say they allow 5mph, car was clocked at 76 so NIP says 71
To me that would be providing faked evidence? Or, at best, tampered?
If that's what they were doing then I would have thought they would simply say "over the speed limit" as that's still factual, just without detail.
For the record: I'm not condoning speeding, just pointing out that while we, as drivers, have to follow certain rules, so do councils etc. I'm genuinely intrigued as to how accurate the cameras are.
If a real, actual human copper clocked me at 71 I'd hope that they would have the sense to say "well, no one was at risk, it's only a tiny bit over - could just be human error for that split second then they went back to 70 so I won't prosecute it". If an automated system clocked me at 71 then I'd like to be 100% sure they were correct and accurate before handing over a hundred quid.
The 10% + 2 guideline was, in my opinion, not just to cover the fact that speedo's don't always read accurately but that a real life copper would generally give some leeway before charging someone with speeding.
Joined: Mar 03, 2006 Posts: 7191 Location: Reading
Posted: Wed Sep 19, 2018 4:37 am Post subject:
For at least a decade now:
Quote:
The UK law is based on the EU standard, with some minor changes.
A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.
So, as I see it, if you're doing say 71 then your speedo is reading at least 71, more than likely a bit more, so they are using that as 'we both know you were speeding'. As I said, I know my Honda is over calibrated by 6%, if I was doing a true 71 then my speedo would be reading just over 75.
What would be interesting would be someone nabbed at 71, 72, 73 who had a DashCam running with a GPS speed readout. _________________ DashCam:
Viofo A119 V3
Joined: Aug 31, 2005 Posts: 15356 Location: Bradford, West Yorkshire
Posted: Wed Sep 19, 2018 10:35 am Post subject:
Kremmen wrote:
So, as I see it, if you're doing say 71 then your speedo is reading at least 71, more than likely a bit more, so they are using that as 'we both know you were speeding'. As I said, I know my Honda is over calibrated by 6%, if I was doing a true 71 then my speedo would be reading just over 75.
True, but that's not always easy to see on an analogue speedo with relatively thick lines and no individual 1mph marks. You'd need to be staring at your speedo constantly to make sure you don't go the slightest bit over 70 - that then becomes dangerous. Probably more dangerous than doing 71-72 mph.
Personally I just stick my cruise control at 65mph (actual about 61) and get much better fuel economy and it doesn't add much time to the journey
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
Posted: Wed Sep 19, 2018 11:11 pm Post subject:
Correct. But if you do some calculations on rolling radius etc. of new v worn out tyres, you will find the difference is negligible. As the tyres wear, the speedo will read faster so you will be going even slower on worn tyres than the speedo reads.
Joined: May 04, 2008 Posts: 754 Location: Knaresborough, North Yorkshire.
Posted: Wed Sep 19, 2018 11:21 pm Post subject:
Yes, a very worn tyre would increase the Speedo display by around 2% (or 1 mph at 50 mph, so quite insignificant really).
[Obviously the GPS displayed speed would be unchanged].
Say the new tyre diameter is around 650mm (this doesn't have to be exact). Then 6mm of tread wear on both top and bottom would take the diameter down to 638mm.
That's a 2% shift in diameter - and diameter is directly proportional to circumference. So the tyre would rev 2% faster and so would the speedo (if driven off the gearbox).
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