>>
>> If you have a digital speedo, then there's no reason why it can't be
>> designed to be accurate to the nearest 1 mph (or maybe even better).
>> However if the gauge is analogue, you've got the stiffness and
>> linearity of the spring that controls the needle, which will vary from
>> one gauge to another due to manufacturing tolerances. I suppose it
>> would be possible to calibrate each speedo by supplying specimen
>> inputs at various spot speeds and adjusting the software that counts
>> the pulses so it gives an accurate reading.
>>
>> Do any modern cars still use mechanical communication (rotating cable
>> in a sheath) between the wheels and the gauge, or do they all use
>> Hall-effect sensors (like bicycle speedos) or light-and-rotating-disc
>> sensors (like computer mice) these days?
>>
>> I checked my speedo against a sign that lights up if you are over
>> speed and it seemed to agree fairly well: the sign was triggered at
>> about 32 mph as shown by my speedo.

>
> I've noticed a difference on my Mondeo between the speedo and the
> average speed readout on the trip computer.
>
> If i set the cruise control at 90mph on the speedo, tomtom is showing
> 85mph, and if i reset the average speed counter whilst the CC is set it
> shows the true speed according to thomas.
>
> If the average speed display knows my actual speed why doesn't the speedo?