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[BazaJT]

I've seen/heard much discussion of late about this idea of zero tolerance on speeding and the use of cruise control to make sure of not falling foul of speed cameras etc. Given that speedometers are not truly accurate where does the cruise take its cue from?Or are cruise speeds just a best guess also?Just curious really as to what tells the system you've hit the speed you've selected.Must admit I've never yet used cruise on any car I've driven with it fitted it's just another useless piece of kit I don't particularly want or need.

[Lizzie Zoom]

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

[Doctor Gollum]

From your last statement I would suggest that you don't understand the purpose of it

But for the first part of your question, all the speed data on a vehicle is provided by the ABS system. The speedometer is the item which is calibrated on police vehicles, or the tachograph/taxi meter on suitably equipped vehicles.

General production vehicles legally have to over read by no more than ten percent, but must over read.

Next time you drive on a motorway,  set your cruise control to 56mph for five minutes and count the number of trucks which pass you... It will be almost every single one, all travelling at 56mph

[Doctor Gollum]

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.

[STEMO]

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.

[Doctor Gollum]

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 

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

[Bigron]

GPS speeds are only that accurate when you are travelling on level roads, DG; they under-declare if you are on an incline, up or down.

Ron.

[Andy H]

GPS speeds are only that accurate when you are travelling on level roads, DG; they under-declare if you are on an incline, up or down.

Ron.

Straight and level. Depending how often your GPS updates it's location it might make a major underestimate of your speed if you are travelling along a winding road.

[Bigron]

Sorry Andy - I meant to mention wiggles as well!
It just emphasises the fact that zero tolerance policing of such an arbitrary thing as a posted speed limit is a crackpot idea, fraught with hazards for the motorist and general public alike.
The only winner will be the government's stealth tax department with its greed cameras. 

Ron.

[Doctor Gollum]

Sorry Andy - I meant to mention wiggles as well!
It just emphasises the fact that zero tolerance policing of such an arbitrary thing as a posted speed limit is a crackpot idea, fraught with hazards for the motorist and general public alike.
The only winner will be the government's stealth tax department with its greed cameras. 

Ron.

Understood re the GPS specifics... Rest of your post is, respectfully, nonsense.

If you don't exceed the posted speed according to your speedo, then you will never be speeding. Simples.

[Bigron]

Is that "respectfully" in the "Yes, Minister" sense ot the term, DG?

Ron.

[STEMO]

GPS speeds are only that accurate when you are travelling on level roads, DG; they under-declare if you are on an incline, up or down.

Ron.

Straight and level. Depending how often your GPS updates it's location it might make a major underestimate of your speed if you are travelling along a winding road.

Straight and level? Like the motorway I did it on then.
Uphill and downhill would make a very small difference, but not enough to make the results void.

[Doctor Gollum]

Is that "respectfully" in the "Yes, Minister" sense ot the term, DG?

Ron.

If it helps

[BazaJT]

You are perfectly correct DG I do not understand the purpose of it[assuming you aimed that bit at me]This then begs the question what exactly is the purpose of it?

[TheBoy]

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

This is why zero tolerance is possible, and has been around for 15+ years - its nothing new.


As DG says, on most ABS equipped cars, the speedo signal usually comes from the ABS ECU.  Non ABS cars, its often from a sensor on the gearbox.