1owner69Elan wrote:The GPS speedometers are certainly not absolutely perfect. Nor are mechanical and electronic ones as they are dependent on tire sizes, transmission, and diff changes. And even if calibrated for a given configuration then changes in tire pressure/temperature/traction can induce error.
That's an interesting post on GPS speedometers.
However, GPS apart, I would not put mechanical and electronic speedometers in the same boat.
As you say, when calibrating a mechanical speedometer, one would work forward from the diff ratio, and wheel and fitted tyre size. You will need to select a specific tyre pressure for your calculation. (As tyre pressure is a function of temperature you do not need to input for temperature). The only way to allow for any changes from the manufacturer specs in tyre, wheel or diff ratios is to change the angle drive gearing, by selecting from a discrete range of ratios.
So although reasonable or close calibration is possible, there is usually a detectable error. Over a life time of different cars, I always would establish the speedo error and be able to do a quick mental calculation if fine results were required for speeds and distances.
The other big disadvantage of mechanical speedos, as I do not need to tell anyone here, is how awkward it can be to replace a faulty cable or angle drive.
With an electronic speedo you do not even need to know the diff ratio, wheel and tyre size, or tyre pressure.
You just drive an accurately measure distance, whether a specific number of wheel revolutions, or a measure mile on the motorway, and input the resulting read-out back into the unit.
If tyres, wheels or the diff is changed, simply repeat the procedure to re-calibrate.
Finding a suitable fixing of the pick-up, normally to the diff input or output is all that is required.
Then fit and forget !
Incidently, out of curiosity I did a sample calculation of the effect of varying tyre pressure on a mechanical speedo read-out.
For a 185/14/70 tyre, dropping a tyre pressure of 2 Bar (29.4 psi ) by 5 psi would mean the speedo reads 1.2% faster, or 0.24% /1psi. Does anyone here drive with their tyres off by anything like 5 psi ?