Originally Posted by CliffordK

I've got a tape that measures in cm. Inches would also work with conversion by one of a multitude of conversion programs or methods.
I set the valve at 0, then slowly roll my tire forward till the valve is down again.

Do it a couple of times, and you have your speedometer/odometer perfectly calibrated to your bike and tire. I round to the nearest half cm,

I usually walk my bike with a little weight on it, but I don't know if that is absolutely necessary.