driving slow : your views ?

nobbysn*ts

Quiet_Spark wrote: »

That may be so, but the rubber on the tyres is wearing all the time which alters the rolling radius.


The values in electronic components can (and do) change over time. For example, if a 100μF capacitor gets a little bit "weepy", it's value can quite easily drop to (say) 80μF and you would never know unless you tested it out of circuit (or an obvious fault was showing).

Not all speedo's read the same either. The speedo on the wifes car (14 month old 2 litre Grand C-Max) shows 60mph yet GPS shows the true speed to be 56mph.
In my Beemer (57 plate 325i Coupe), when the speedo shows 60mph the GPS shows 58mph.


One thing is guaranteed though, and that is all car speedo's over-read rather than under-read which probably explains why people get peed off when someone religiously sticks to 30 mph on the speedo when they could well be doing 26mph in reality.

And they could well be doing 30mph. As I said, most speedos are digital, a weepy capacitor would have little effect on a digital, rather than analogue system. So if you're behind me, following me at a shown 30mph on my speedo, I'm not doing 26. And if your speedo is slowing 30 as well, you're going to have to accept it's correct, and not try to get up to 34mph just in case. As you say, at an indicated 30, you're doing 28 and 29 in reality. You could chose to try to get away with the extra 3mph, if you insist on driving at an indicated 34mph. I don't, so if you're behind me, you're be doing less than you want to, but still close enough to the limit.

nobbysn*ts

veryoldbear wrote: »

Just trying to put some figures on this tyre wear thing. The old bus outside has 215/55R16's on it, which gives me a nominal rolling diameter of about 650mm. If I wear down about 10mm of tread (for argument), the diameter would be 630mm and I reckon that would make about 3% difference on the speedo or say 2mph at NSL70.

Say the nominal 16 inch rim, or 406mm, add on the 236mm sidewall, Yes about 640 mm. 10mm of tread, (7mm to 2mm) 10mm in 640mm is less than 1.56%, or 1.1 mph at 70mph. Which is about right, as mine is reading about 68 to 69 on the sat nav, and it's due new tyres. So I would expect the satnav to increase slightly.

veryoldbear

nobbysn*ts wrote: »

Say the nominal 16 inch rim, or 406mm, add on the 236mm sidewall, Yes about 640 mm. 10mm of tread, 10mm in 640mm is less than 1.56%, or 1.1 mph at 70mph. Which is about right, as mine is reading about 68 to 69 on the sat nav.

10mm on the tread is 20mm on the diameter, so 3% or 2 mph

nobbysn*ts

veryoldbear wrote: »

10mm on the tread is 20mm on the diameter, so 3% or 2 mph

All my tyres started new at about 7mm, and wear down to 2. Never had any that started at 12mm. The speedo would be calibrated for a standard tyre at new, so anything over the nominal 7mm would throw it out and make it read low.

Cornucopia

All this tyre discussion is all very well. But for 30mph zones, it's a bit academic. There is a minority of drivers who for various reasons are doing 25-28mph. A few doing about 30. And the majority who interpret 30 as 33-36mph. I don't see the Police taking nearly the same interest in this speeding as that on faster roads - although there's no obvious reasons why they shouldn't.

nobbysn*ts

Maybe because they see what happens at 80mph, and what happens at 26mph?

Cornucopia

So you are saying that they actually believe the "speed kills" narrative?

nobbysn*ts

Cornucopia wrote: »

So you are saying that they actually believe the "speed kills" narrative?

Me? I believe in inertia. As to your question, you'll have to check with someone that's crashed at 80, and then again at 26. Or maybe you'll have a better chance of an answer if they've done it the other way round? Well at least after the first crash anyway, after the second they'll probably be equally responsive.

dori2o

Cornucopia wrote: »

All this tyre discussion is all very well. But for 30mph zones, it's a bit academic. There is a minority of drivers who for various reasons are doing 25-28mph. A few doing about 30. And the majority who interpret 30 as 33-36mph. I don't see the Police taking nearly the same interest in this speeding as that on faster roads - although there's no obvious reasons why they shouldn't.

Probably because they are not travelling at the speeds being displayed by the speedo, AND because most local authorities apply tolerances to the speed limits because of the fact that speedos are not accurate.

For example, our LA has the cameras set at speed limit + 10% + 4mph, so in a 30 zome the camera's, fixed and mobile, are set at 30 + 3 + 4 = 37mph.

Looking at other LA's in this area they seem to apply the same rules.

Therefore people travelling 33-36mph are not committing an offence.

dori2o

nobbysn*ts wrote: »

Nope. The majority of cars still have the wheels the were made with. The speedo is digital, with a sensor driving a stepper motor, so it's as precise as the day it was made. Mechanical speedo from years ago are the exception now. And as all mine agree with the sat nav to within a mile or two at most, at 70mph. I'm happy to confirm it's now an urban legend that went out with the cable from the gearbox.

Not correct, most speedo's are calibrated with out the wheels on, but the way it's done is to calibrate as if a certain defined diameter wheel and tyre is fitted.

Therefore if the garage or owner changes the diameter of the wheel, or even the depth of tyre sidewall, the reading will be out, The reading will also change as the tyre wears out, and also it changes if the tyre isn't properly inflated as this again changes that defined total diameter.

Electronic components, just like mechanical, lose their efficiency over time, but to a lesser degree, there is still a loss of accuracy as time goes by.

Clearly it isn't, and is the very reason why all LA's/police authorities apply tolerances to their speed camera's, both mobile and fixed.