When doing a manual calibration for a tachograph we used 10 wheel turns and
ensured the transmission slack is NOT included. Put a chalk line from the tyre
onto the road and drive or push the car in one direction only until the chalk
mark has rotated 10 times and is back at the same angle when hitting the road
surface. Mark the road at the end point. Do not push the vehicle back to mate
the chalk mark and line up, rather push it forwards to make it 11 turns. Then
divide the road distance between the marks by the number of wheel turns. This
is accurate enough to use in Forensic Calibration and Court use.
The circumference of an inflated tyre is not the distance it travels along a
road so do not just measure your spare tyre!
Be aware that tyre pressures and tread depths that differ from the time you
calibrated your device will affect its accuracy - but a normal speedo is
subject to the same variations.
Guy R Day
Glen Byrns <grbyrns@ucdavis.edu> wrote:
I was just going over the forum on the "Flying A" website for Austin A30 and
A35 owners and found a GREAT tip many Spridgets can use.
One A35 owner frustrated with the inaccuracy of his speedometer after changing
out his differential figured out a smart and "we tight" method to put an
accurate speedometer, odometer, tripmeter etc into a car.
He mounted a bicycle cyclometer on the dash, made up a simple extension wire
to reach back to the rear wheel where he mounted the sensor. He then
hot-glued the magnet to the wheel in alignment with the sensor. Follw the
calibration technique given in the cyclometer instructions, punch in the
calibration number and away you go! You should be able to get a decent one
for about $35-$60.
I've used them on my bikes for years and I can tell you that they are far more
accurate than the average car speedo/odo. If you do the calibration carefully
you can get them to +/- 1% accuracy. One trick not mentioned in the set-up
instructions is to have someone else push the car with you in it for two or
three revolutions of the measured wheel. Then divide the resulting distance
by 2 or 3 to get a very accurate calibration number. I'm going to put one on
the bugeye right away and go get another one for the Austin.
Hey Ed, Gerard, Jacques, how about putting this tip on your web pages?
Regards,
Glen Byrns
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