> You're correct, of course, but I think it's difficult to come up
> with a reliable allowance for that "flat bit at the
> bottom". These factors could include the pressure the tyre is inflated
> with, the load in, (and of), the vehicle
> itself, and the variable of how much stiffness the maker builds into the
> tyre walls.
Unfortunately, to come up with an answer correct enough to base an accurate
speedometer upon, you have to do like the bicycle folks and mark where your
tire hits the ground, roll it one complete revolution, and measure the
distance between marks. "Where the rubber meets the road" as they say.
That is why, as part of my rear axle project, I am incorporating a Sigma
Sport 1200C bicycle speedometer pickup inside the brake drum. It will handle
speeds up to 183mph with more accuracy than any of the mechanical speedos
other than rally units. No. I do not know why "183". A lot of motorcycles
use them, too. I will have the stock speedo for ordinary use and the Sigma
for when I want to know reality.
David Lieb
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