All those effectively make the speedo read higher than the true speed. So even more reason for having it as near correct as possible with new tyres, etc. Not starting out with it close to the upper limit.
I prefer to read stuff from those that actually know.
Saving a lot of wasted time, discussing things with those that don't know.
'To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ?high? by a certain amount.'
Very good article that - and written just for you. No need for boring figures - 'a certain amount' is just fine. Very scientific.
But tell us of your findings, Dave. What is that certain amount on your car? And on all the others you've owned?
No it wasn't why would it be.
couldn't care less .
IU have no finding to report.
What is that certain amount on your
The differences in wheel diameter resulting from the above circumstances [eg wear] could be tiny (maybe a few millimetres), but at 30mph your car wheels are rotating 6-7 times every second, so it can quickly make a difference of a few miles per hour.
"Those that actually know." Riiight.
Cheers
whisky-dave used his keyboard to write :
That is a very dumbed down version and in places wrong.
Decreasing the tyre pressure makes no difference to the rolling diameter, unless the pressure is so low as to allow the tread to fold over itself. Tyre wear can account for around 1% inaccuracy, which is not 'several MPH.
The accuracy of GPS/ satnav only varies by a tiny amount on hills, not nearly as much as the accuracy loss due to bends, which themselves really not that significant.
I have never come across any satnav which takes into account hills, in its speed calculations - as some are suggested to in that article.
Not that expert really!
And is the A33 a motorway?
But does change the radius where the tyre contacts the road and it is that which determines the rotation rate of the wheel for a particular road speed and the speedo uses the wheel rotation rate.
Which is what determines the speedo reading.
How do those indirect tyre pressure monitor systems that work by comparing the distance a wheel has travelled compared to the other wheels and what it was doing before do it then? My car has such a system and it lets you know that a tyre has lost some pressure a long time before the pressure has got so low the tread is folding over.
G.Harman
Off course it isn't , I was simply pointing out there are some other places where it is possible to find roadside markers that enable people to check their speed over a measured distance.
Sorry if you feel adding to a thread has diverted attention from your Smart Alec comment . If you wanted the last word you should have said so.
G.Harman
The Natural Philosopher has brought this to us :
I understood the point you were making, I just thought it needed ore emphasis :D
The Natural Philosopher has brought this to us :
Moving coil can be much more accurate than that, remember we used moving coil for accurate measurements for many years?
My last two cars have used a stepper motor system to drive the needle. They self calibrate by blindly driving the pointer needle to the stop, then assume the needle follows the motor movement there after.
No reason why that type should have any detectable error at all in the reading.
The Natural Philosopher explained :
I would argue that modern speed indicating hardware is quite capable of displaying the speed perfectly accurately. They just insert a deliberate high reading, because they can get away with it and it gives their customers a bit of wobble room.
I agree, if you want to check the accuracy of your speedo, just follow a lightly loaded lorry on the motorway on the level. They will be foot hard to the floor, running on the limiter at a precise 56mph in most cases. Isn't the actual limit 60mph though?
Are you saying that the wheel and tyre are rotating at different rates?
So for each revolution of the wheel the car will move forward by a distance equal to the circumference of the tyre. The circumference remains constant whatever the pressure so the ratio if vehicle velocity to rotational speed of the wheel remains the same regardless of tyre pressure.
Yet the radius clearly is reduced where it touches the road.
That's an interesting claim. So in a hypothetical limit case, in which the tyre circumference is very close to the wheel at the bottom because the weight of the car is on it, and far away from the wheel at the top, both circumferences would rotate at the same rate as they are connected by the tyre, as you imply. However the wheel would be effectively rolling on the ground and would require more rotations to cover the same distance than if the tyre was fully inflated. In my hypothetical case, the outer perimeter is not rigid, but varies its speed in parts to keep up with the wheel, although its circumference may well be constant.
I have never heard of such a system.
What car, and what grounds do you have for claiming that the pressure loss is detected by differential rotation.
Seems like just going round in circles would set that one off..
No.
Tell me, what is the rolling radius of a tank track?
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