> On Mon, 15 Sep 2008 05:29:39 -0700 (PDT), Boltar
> yahoo.co.uk> wrote:
>

>> Since truck, bus and train speedos can be accurate why can't they do
>> the same for cars? It can't be hard in these days of electronic
>> everything to have some optical revolution counter on an axle so whats
>> the problem? I drove past one of these flash you speed up signs the
>> other day and while my speedo said 38 the sign said 34 so thats a good
>> percentage out. And my tyres and new and inflated to the correct
>> pressure so they're not the culprit. So why don't car manufacturers
>> bother?

> I suppose one reason is that the radius, or rather, the circumference
> of the wheels of cars/lorries etc aren't constant. It will change as
> the tyre wears down and as the pressure of the air inside it
> expands/contracts/leaks.
>
> Perhaps a different means of measuring the speed of a vehicle is
> needed - perhaps some sort of radar signal bounced off the road below
> the vehicle?