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Author Topic: Curiosity question  (Read 896 times)

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Lizzie Zoom

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Re: Curiosity question
« Reply #15 on: 11 February 2018, 10:57:48 »

As I understand it the cruise control works in conjunction with the speedo.  As the latter is not calibrated the former is also technically inaccurate. As your tyres wear down and your system ages that must have an effect, slight maybe, but still a factor that makes the speedo and cc inaccurate compared say to a police calibrated speed monitoring device.


This, to me, is why there cannot be zero tolerance with speed limits under 10% as of now ;)
Actually the fact that all production speeds are legally obliged to overread, provided your speedo doesn't exceed the posted limit, you can never be caught speeding...

Except by the fluke of physics as displayed in that video posted last week where the chap who never exceeded 72mph yet got pulled for 110 because the police failed to spot the car which the radar had actually picked up.

Thanks DG.  I didn't know that and that is useful information :y :y
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Doctor Gollum

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Re: Curiosity question
« Reply #16 on: 11 February 2018, 22:01:58 »

Cruise control is best suited for open motorways... Proper distance driving... ie running up to Scotland late at night, or hacking across the continent ;)

In the UK, people who get the most benefit are lorry its and coach drivers... Whose speed is artificially restricted as it saves holding the accelerator down for four hours.

In a car, it's handy for regulating vehicle speed in restricted roadworks etc. If fitted, I tend to use it alot as it makes the journey smoother. Changes in traffic flow can easily be dealt with by a mere push of a button.  :y
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ronnyd

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Re: Curiosity question
« Reply #17 on: 11 February 2018, 22:15:32 »

Since my being "flashed" late one night in a 30 zone and the subsequent speed awareness course, i use it quite a lot in built up areas. ;)
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aaronjb

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Re: Curiosity question
« Reply #18 on: 12 February 2018, 09:13:41 »

Older cars were 10% tolerance on the speedo, I think newer ones have to be something like 2%, but in all cases, overread.

According to the IVA (which is, for the most part, the same as type approval) the tolerance is "(true speed/10) + 6.25" along with a table for those incapable of maths.

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Fuse 19

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Re: Curiosity question
« Reply #19 on: 12 February 2018, 09:28:18 »

I downloaded a GPS speedometer app. At 70mph my car is doing 65-66. I tried it with this supposed 10%+1.5mph =78.5mph. I would need an indicated speed of 84 to be doing that.
So, theoretically, I could sail past all of the cars in lane 2, doing a good 10mph more, and still be ok.
Precisely  :y

GPS speeds are more accurate than 10%, typically 2% due to the signal delay ;)

Its nothing to do with signal delay, its down the accuracy of the position determined on a moving object which will have typically a 1-2m inaccuracy, GPS often reads slightly low  :y
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Kevin Wood

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Re: Curiosity question
« Reply #20 on: 12 February 2018, 09:28:56 »



Ahh, yes. Used to have most of that manual burned into my brain.

The point being the speedo can over-read , but if it under-reads by any amount at all, you fail.

By the way, I assume you've seen this gem?

https://www.gov.uk/government/consultations/road-vehicles-improving-air-quality-and-safety

Time to get the Cobra finished!
« Last Edit: 12 February 2018, 09:30:29 by Kevin Wood »
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Fuse 19

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Re: Curiosity question
« Reply #21 on: 12 February 2018, 09:33:40 »

As I understand it the cruise control works in conjunction with the speedo.  As the latter is not calibrated the former is also technically inaccurate. As your tyres wear down and your system ages that must have an effect, slight maybe, but still a factor that makes the speedo and cc inaccurate compared say to a police calibrated speed monitoring device.

Its actually even less high tech, the ABS ecu produces a pulse count on the non drive by wire cars and speed value (its not in MPH or KMH) which goes to the cruise controller (non drive by wire) or the ECU (drive by wire) AND the instruments so the two are independent.

What happens is you activate cruise based on what the speedo is indicating and the ECU/cruise controller simply opens or closes the throttle to match the selected speed value (or adds or reduces fuel if it is a diesel)

So in summary, the cruise system is only inaccurate due to the reading of the speedo, add a more accurate speed indication and the cruise will naturally be more accurate.
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aaronjb

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Re: Curiosity question
« Reply #22 on: 12 February 2018, 11:41:51 »

By the way, I assume you've seen this gem?

https://www.gov.uk/government/consultations/road-vehicles-improving-air-quality-and-safety

Time to get the Cobra finished!

Yeah, reading up it seems that it's likely to come into force on 01/01/19, so if it hasn't been IVA'd by then I'll be bollarded (well, maybe not, I know someone who regularly gets carb'd FEs through an emissions test with CATs in place)..
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Kevin Wood

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Re: Curiosity question
« Reply #23 on: 12 February 2018, 11:44:08 »

Yes, I'm sure there are ways and means, but it's a ballache that you don't need. >:(
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aaronjb

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Re: Curiosity question
« Reply #24 on: 12 February 2018, 11:52:48 »

Indeed! Seems some (or most) of the manufacturers aren't that bothered either with their responses being "fit a modern engine like an LS/ecoBoost/etc*" *delete as applicable depending on if you're talking to a Cobra replica manufacturer or 7-clone manufacturer..
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Field Marshal Dr. Opti

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Re: Curiosity question
« Reply #25 on: 12 February 2018, 12:01:32 »

Speaking of GPS accuracy Mrs Opti bought me one of these to play with a few years ago. Cost around 450 if memory serves.

Extremely accurate. :y 



http://racelogic.co.uk/_downloads/Techical_Specs/PERFORMANCE%20BOX%20-%20Technical%20Specs.pdf
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Re: Curiosity question
« Reply #26 on: 12 February 2018, 13:17:59 »

Which Mrs Opti ?  ??? :P ;D
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Fuse 19

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Re: Curiosity question
« Reply #27 on: 12 February 2018, 13:18:25 »

That is actually pretty inaccurate from a position perspective, the clever bit is the G-meters  :y

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jimmy944

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Re: Curiosity question
« Reply #28 on: 12 February 2018, 13:20:34 »

Indeed! Seems some (or most) of the manufacturers aren't that bothered either with their responses being "fit a modern engine like an LS/ecoBoost/etc*" *delete as applicable depending on if you're talking to a Cobra replica manufacturer or 7-clone manufacturer..

So they've dropped this neatly into the "too difficult to fix" box then  ::)
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Field Marshal Dr. Opti

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Re: Curiosity question
« Reply #29 on: 12 February 2018, 13:26:48 »

Which Mrs Opti ?  ??? :P ;D

Let me think on that. Number 2 , I think. :)
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