Speedo Calibration?

[mike.perry]

Yes I had thought of all those methods. I couldn't decide whether the rolling radius, ie the distance between 2 chalk marks would be much different from the tyre circumference since when the car is pushed along the road the whole of the circumference of the tyre comes in contact with the road regardless of tyre pressure.
I could also have measured the radius from the wheel centre to the ground and multiplied by 2 x 3.141592654 or alternatively pressed the pi button on the calculator, but I felt that would be less accurate.

[bmcecosse]

Don't think the 'radius' method(s) would work - since the tyre squishes down you may not get an accurate measurement.