Nick McCormack wrote:
>
> Not sure re different Speedo's for O/D etc but was having a problem with
> mine in that it was over reading i.e. saying that I was going faster and
> travelling further than I was.
>
> I have since sent it off for recalibration to a firm in SE England (UK).
> What I had to do was measure the tyre circumference, remove the speed, jack
> up the rear wheel, turn it twelve times and count how many times the speedo
> drive turns itself.
From my experience with bicycle computers, you have to
measure the tire circumference by rolling the tire and
making chalk lines, not by wrapping a tape measure around
it when jacked up.
Because of the flat contact area at the bottom, the
circumference of a rolling wheel is a fair bit less than
the circumference of an inflated tire.
Put a big gob of chalk across the tread, roll the car
slowly for at least one revolution, and measure the
distance between chalk lines.
This should be pretty accurate for low speeds. At
high speeds the tire circumference grows a little, but
only meaningful at Thrust SSC speeds. ;>
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
"Have some riot gear. On the house!" - Chief Wiggum
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