Cold & mobile cameras
Discussion
I got a fresh SP30 after 20yrs. I was just keeping up with traffic.
‘’Everyone here is innocent” shawshank redemption.
It’s a good thing too, I was beginning to be a bit of an Audi driver. I’m quite pleased really, I entered the cold snap with a really light right foot and the M5 is a fairly decent car in the cold if you drive it with virtually no throttle. Had I not been given a ticket so recently I reckon I would have misjudged the cold and got it really wrong. So I suppose thanks to the system.
Meanwhile these new openly published traffic speed tolerances and huge fines....does anyone know what the accuracy of a bmw speedo is? I had to use a 30mph zone late at night today on a 60 mph patch. I did the speed required and had a queue from hell behind me. What can you do...??
‘’Everyone here is innocent” shawshank redemption.
It’s a good thing too, I was beginning to be a bit of an Audi driver. I’m quite pleased really, I entered the cold snap with a really light right foot and the M5 is a fairly decent car in the cold if you drive it with virtually no throttle. Had I not been given a ticket so recently I reckon I would have misjudged the cold and got it really wrong. So I suppose thanks to the system.
Meanwhile these new openly published traffic speed tolerances and huge fines....does anyone know what the accuracy of a bmw speedo is? I had to use a 30mph zone late at night today on a 60 mph patch. I did the speed required and had a queue from hell behind me. What can you do...??
Iandk said:
...does anyone know what the accuracy of a bmw speedo is? I had to use a 30mph zone late at night today on a 60 mph patch. I did the speed required and had a queue from hell behind me. What can you do...??
A GPS provides an accurate speed measurement if you're driving at constant speed and where it has a decent view of the sky. Speedos generally read a few mph low.My M4s speedo runs pretty much exactly 2 mph fast.
I run cameralert on my iphone x with the screen in standby...so it just chirps up when a camera is in range and facing you.
Cameralert has been around for years and the camera positions are crowdsourced...subscription based but find a new camera and get free subs for life.
I run cameralert on my iphone x with the screen in standby...so it just chirps up when a camera is in range and facing you.
Cameralert has been around for years and the camera positions are crowdsourced...subscription based but find a new camera and get free subs for life.
AW10 said:
A GPS provides an accurate speed measurement if you're driving at constant speed and where it has a decent view of the sky. Speedos generally read a few mph low.
Are you quite sure that's correct? I've never seen a speedo read a few mph low? I've tested most of my recent cars with GPS and the speedo reads around 2 mph faster.JMBMWM5 said:
I have one of these, it works well.
https://cheetahgps.co.uk/product/cheetah-c550-blac...
Is Waze not a better option (and free)?https://cheetahgps.co.uk/product/cheetah-c550-blac...
Andy M said:
Is Waze not a better option (and free)?
I've running waze alongside cameralert and Waze is not nearly as reliable for speed camera positions...but once fully ported onto google maps it might get better with time.The cameralert database goes back over 15 years from when tomtom nav software became available for pocket PCs and users of pocketgpsworld started collecting camera locations as POIs and adding them to the Nav software. Nowadays you get a database update most days. But you have to have a subscription...not something everyone is prepared to pay.
Yup all Speedos over read. It's the law. That way if you get caught doing 35 your car prob said 38 or something so no excuse! And BMWs are renowned for larger errors. My 06 330i used to read 100 at a "real" 92. Difference between ban and points.
As well as the "fixed" error programmed in to the reading, your Speedo error is tyre tread depth / pressure dependant as the speedo is transmission / wheel speed driven (electronically these days)
Over read error gets larger the more your tyres are worn (or tyres underinflated too, more so)
Worn or low pressure tyre or heavily loaded car (eg 5mm tread difference or 10mm diameter difference (on an 18" low profile) might give say 2 to 3% extra error. (2 or 3mph at 100mph) depending on tyre diameter. That's how ABS based TPMS systems work, as a tyre presssure goes down the wheel speed increases, it will detect even 2 or 3 psi low easily.
Not massive but if you're riding the error at cameras it all counts. Cameras measure real speed not what your Speedo said!
This creates the interesting problem that your odometer (which ignores factory set speedo error) will also over record mileage with worn or low pressure tyres!
As well as the "fixed" error programmed in to the reading, your Speedo error is tyre tread depth / pressure dependant as the speedo is transmission / wheel speed driven (electronically these days)
Over read error gets larger the more your tyres are worn (or tyres underinflated too, more so)
Worn or low pressure tyre or heavily loaded car (eg 5mm tread difference or 10mm diameter difference (on an 18" low profile) might give say 2 to 3% extra error. (2 or 3mph at 100mph) depending on tyre diameter. That's how ABS based TPMS systems work, as a tyre presssure goes down the wheel speed increases, it will detect even 2 or 3 psi low easily.
Not massive but if you're riding the error at cameras it all counts. Cameras measure real speed not what your Speedo said!
This creates the interesting problem that your odometer (which ignores factory set speedo error) will also over record mileage with worn or low pressure tyres!
Edited by mikeN54 on Monday 4th February 20:13
I've reworked my figures on paper rather than in my head.
So for a 680mm diameter michelin tyre with ~7.6 mm total tread depth.
At legal minimum with 1.6mm tread the diameter is then 668mm.
Thats a % decrease in size from 680 to 668 of 1.765%
At 70 mph thats a difference of 1.25mph.
If the speedo takes its reading from the speed of the rear wheel then as the rear tyres reduce in circumference they have to make more revolutions to cover the same distance therefore the tendency for a speedo to over read increases. ie if the speedo was correct with new tyres on my car at 70mph it would show 71.25mph with tyres at the legal minimum.
So for a 680mm diameter michelin tyre with ~7.6 mm total tread depth.
At legal minimum with 1.6mm tread the diameter is then 668mm.
Thats a % decrease in size from 680 to 668 of 1.765%
At 70 mph thats a difference of 1.25mph.
If the speedo takes its reading from the speed of the rear wheel then as the rear tyres reduce in circumference they have to make more revolutions to cover the same distance therefore the tendency for a speedo to over read increases. ie if the speedo was correct with new tyres on my car at 70mph it would show 71.25mph with tyres at the legal minimum.
The most recent figures of 1.765% and 2.35% both use twice the treadwear amount to calculate the difference in diameter; using the treadwear amount just once is wrong. The two figure differ because of different treadwear amounts (6mm versus 8mm).
The rolling diameter also expands slightly with speed. And as an aside I seem to recall that aircraft tyres can grow up to 5 percent in diameter after 50 heat cycles; wonder if the same happens to automobile tyres?
The rolling diameter also expands slightly with speed. And as an aside I seem to recall that aircraft tyres can grow up to 5 percent in diameter after 50 heat cycles; wonder if the same happens to automobile tyres?
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