Cycling UK Forum

thirdcrank wrote:A belated apology for having inadvertently revived the speedo accuracy debate when I was only trying to answer MickF's OP.

I've just checked - there's an r in the month, so it's due

thirdcrank wrote:

pwa wrote: ... I use a satnav to get a more accurate reading, though, as you say, you have to be careful about lag. And that gives me a feel for how far my speedo is below my actual speed. 10% or so.

Are you sure about that because the waggle room is the other way? ie it's OK to have a speedo that overreads by up to 10%, but any underread is illegal these days.

They are very accurate at a steady speed, so are best used to 'calibrate' ones own speedometer so the underread is known.

In any event, it supplements, not replaces the speedo.

pwa wrote:I'd just like a speedo that tells me I'm doing 25mph when I am actually doing 25mph. A speedo that gives me a figure that is not correct is wrong, and I would have expected something better by now. As it is, we all go round knowing our speedos are overstating our true speed and doing mental arithmetic to get a more accurate figure. I suppose that might be good mental exercise though.

Well, except that the same 25 mph on a nice summer day might read as 27 mph on a cold, frosty day. Which one do you want to be right?

edit: yes, that's a slight exaggeration, but variations of more than 2 mph can be caused by varying conditions

Vorpal wrote:

pwa wrote:I'd just like a speedo that tells me I'm doing 25mph when I am actually doing 25mph. A speedo that gives me a figure that is not correct is wrong, and I would have expected something better by now. As it is, we all go round knowing our speedos are overstating our true speed and doing mental arithmetic to get a more accurate figure. I suppose that might be good mental exercise though.

Well, except that the same 25 mph on a nice summer day might read as 27 mph on a cold, frosty day. Which one do you want to be right?

edit: yes, that's a slight exaggeration, but variations of more than 2 mph can be caused by varying conditions

How would that be?

thirdcrank wrote:

pwa wrote: ... I use a satnav to get a more accurate reading, though, as you say, you have to be careful about lag. And that gives me a feel for how far my speedo is below my actual speed. 10% or so.

Are you sure about that because the waggle room is the other way? ie it's OK to have a speedo that overreads by up to 10%, but any underread is illegal these days.

Quite right. I meant above, not below,

pwa wrote:I'd just like a speedo that tells me I'm doing 25mph when I am actually doing 25mph. A speedo that gives me a figure that is not correct is wrong, and I would have expected something better by now. As it is, we all go round knowing our speedos are overstating our true speed and doing mental arithmetic to get a more accurate figure. I suppose that might be good mental exercise though.

"all"?

I don't and tbh I don't care.
I trust the speedo not under read and that's good enough for me. Beyond that driving to conditions is far more important than getting into a flap that I might be able to go a few mph faster in the infrequent gaps.
Holding a constant speed results in far better fuel efficiency than all the accelerating and braking that passes for vehicle craft these days.

kwackers wrote:

pwa wrote:I'd just like a speedo that tells me I'm doing 25mph when I am actually doing 25mph. A speedo that gives me a figure that is not correct is wrong, and I would have expected something better by now. As it is, we all go round knowing our speedos are overstating our true speed and doing mental arithmetic to get a more accurate figure. I suppose that might be good mental exercise though.

"all"?

I don't and tbh I don't care.
I trust the speedo not under read and that's good enough for me. Beyond that driving to conditions is far more important than getting into a flap that I might be able to go a few mph faster in the infrequent gaps.
Holding a constant speed results in far better fuel efficiency than all the accelerating and braking that passes for vehicle craft these days.

Where I live and do most of my driving is not particularly densely built up and there are often occasions where the road ahead is clear, conditions are good enough to do the speed limit but there is a limit of, say, 30, and I don't want to go over. But I have a lot of jobs to do so don't want to waste time either. Believe it or not, I find it useful to know with reasonable accuracy what speed I am doing. And of course I don't drive to a limit regardless.

You can also add to that the fact that I just like stuff to work properly, and the cack speedos we have at the moment belong to another age.

pwa wrote:If and when a vehicle manufacturer comes up with a speedo that works and actually tells you what speed you are doing (imagine that!) all other speedos will look out of date. There will be a rush to catch up. There is no benefit to inaccurate information. It helps nobody.

There must be some around.

In the old days, before sophisticated speed traps and cameras, the commonest way of getting caught for speeding was to have a police car follow the suspect, maintaining the same speed as their car for a certain distance (0.2 miles was it?). That was enough to secure a conviction. But the prosecution would have fallen flat if it could be proved that the police car's speedo was incorrect. Ergo, it couldn't have been...

What I do know is that the speedo on my bike is pretty accurate - because I've calibrated it myself. Pity I can't 'prove' its veracity by playing little games with the BiB. Even if there were speed limits for bicycles*, I'm not fast enough....

*Yes I know there are cycle speed limits in some restricted areas, like Richmond Park. Not cycled there recently.

661-Pete wrote:In the old days, before sophisticated speed traps and cameras, the commonest way of getting caught for speeding was to have a police car follow the suspect, maintaining the same speed as their car for a certain distance (0.2 miles was it?). That was enough to secure a conviction. But the prosecution would have fallen flat if it could be proved that the police car's speedo was incorrect. Ergo, it couldn't have been...

VASCAR was the system I remember.
It used a timer rather than the speedo and had it's own odometer although it could also be manually triggered using squares painted on the roads for the purpose (don't see those anymore).
I think it might still used.

kwackers wrote:

661-Pete wrote:In the old days, before sophisticated speed traps and cameras, the commonest way of getting caught for speeding was to have a police car follow the suspect, maintaining the same speed as their car for a certain distance (0.2 miles was it?). That was enough to secure a conviction. But the prosecution would have fallen flat if it could be proved that the police car's speedo was incorrect. Ergo, it couldn't have been...

VASCAR was the system I remember.
It used a timer rather than the speedo and had it's own odometer although it could also be manually triggered using squares painted on the roads for the purpose (don't see those anymore).
I think it might still used.

That didn't come into large scale use until the seventies and not in every police vehicle even then. Police cars had accurate and calibrated speedometers for years before that. Police motorcycles continued to be fitted with accurate chronometric speedometers long after most other bikes had the cheaper and less accurate magnetic - so called eddy current speedos fitted.

pwa wrote:Where I live and do most of my driving is not particularly densely built up and there are often occasions where the road ahead is clear, conditions are good enough to do the speed limit but there is a limit of, say, 30, and I don't want to go over. But I have a lot of jobs to do so don't want to waste time either.

But 10 miles at a steady 30mph instead of 27mph will save you a whole 2 minutes 13 seconds & that is only if you can maintain a steady speed. If you are doing any slowing down & speeding up in that distance any time savings will be much reduced.

pwa wrote:You can also add to that the fact that I just like stuff to work properly, and the cack speedos we have at the moment belong to another age.

There is something like a 2% variation in overall diameter between a new tyre & one worn to the legal limit. Our car lists 2 pressures "Comfort" & "Eco" with around a 15% difference (35 v 41 psi for the front tyres). That will make a difference to the effective rolling diameter (but I've no idea how much). Tyre make & model will probably make some difference too. How do you propose a non "cack speedo" will take those into account to give an accurate reading?

Once upon a time, the calibrated speedo's of patrol cars were checked with a stop watch over a measured mile. I've no idea about what happens now. Here's the start of the measured mile on Otley Road in Baildon, not a million miles from where EB used to be until quite recently: in the days when I was concerned about such things I've checked a Cateye Mity 2 by riding up and down it several times, only to find out that after several minor adjustments I was back at the default setting. And FWIW, there are speed cameras along there now.

https://www.google.co.uk/maps/@53.84366 ... 384!8i8192

A bit like me, it's showing its age.

Look away now if you don't want to see what's become of the shop.
https://www.google.co.uk/maps/@53.83555 ... 384!8i8192

RickH wrote: How do you propose a non "cack speedo" will take those into account to give an accurate reading?

Laser? Backed up by GPS. Something like that. Something that does not rely on a crude estimate of the current diameter of a tyre. I have used laser technology to survey when installing extra long field gates at a golf course, so I'm sure a big car company could work on that. It is a matter of measuring how distance between the car and a fixed point changes over time.

pwa wrote:

RickH wrote: How do you propose a non "cack speedo" will take those into account to give an accurate reading?

Laser? Backed up by GPS. Something like that. Something that does not rely on a crude estimate of the current diameter of a tyre. I have used laser technology to survey when installing extra long field gates at a golf course, so I'm sure a big car company could work on that. It is a matter of measuring how distance between the car and a fixed point changes over time.

What's the point of going to that trouble and expense. WHat we have now is adequate for it's intended purpose ie to enable people to keep within speed limits.

pete75 wrote:

pwa wrote:

RickH wrote: How do you propose a non "cack speedo" will take those into account to give an accurate reading?

Laser? Backed up by GPS. Something like that. Something that does not rely on a crude estimate of the current diameter of a tyre. I have used laser technology to survey when installing extra long field gates at a golf course, so I'm sure a big car company could work on that. It is a matter of measuring how distance between the car and a fixed point changes over time.

What's the point of going to that trouble and expense. WHat we have now is adequate for it's intended purpose ie to enable people to keep within speed limits.

How much would it cost to combine a GPS reader with a normal speedo so that the GPS reader re-calibrates the speedo based on comparisons between the two at times when the GPS is likely to be accurate? Next to nothing by the time you have put it in thousands of new vehicles. And it would just bring the antiquated speedo more into line with other measuring devices we have in modern society. Why should we make do with something so crude and inaccurate when we can do better?

The other measuring device that winds me up with its crude inaccuracy is bathroom scales...