Speed Awareness Course

[Vorpal]

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?

The contraction of tyres and air pressure on a cold day can change the pressure as much 10% versus a summer day (the rule of thumb is 2% per 5 degrees C; that's not perfect, but it works pretty well). The cold will reduce tyre flexibility. The cold, frosty road surface will have reduced friction. The combination will result in a changed speedometer reading. 2 mph is my estimate, probably conservative, based upon experience 30 years or so ago.

I will see if I can find documented evidence when I get a chance.

[661-Pete]

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.

I remember there was a measured mile - and a measured ½mile - set up along Thornton Road in Bradford, too. When I was living there was long before the days of cycle computers - but I remember using it to get some idea of estimating distance by counting pedal-turns...

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

NO!!!! Tell me it ain't so!

Aha - done some googling, they seem to be still trading albeit in a smaller shop round the corner. Better than total oblivion I suppose...

[kwackers]

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...

I suspect the truth is you're in a minority. Most people simply don't care.
For it's intended use your average speedo is fit for purpose, particularly as most folk don't pay it any attention at the best of times.

[661-Pete]

When I'm driving in France, I try to check my speedo against the radars pédagogiques - non-enforcement illuminated signs which display your supposedly measured speed - which are scattered along French roads, far more numerous than they are in the UK. Unfortunately those don't appear to be very accurate, either. At least, I get wildly inconsistent results from them.

I must assume that the enforcement radars in France - also very numerous - are accurate. At any rate, I haven't been zapped - yet!