Originally Posted by
noisebeam
Keep in mind if doing the roll out method and you want to be accurate you need to be fully seated on the bike in riding position with tires inflated to your normal pressure.
Why? Does the rubber in the center of the tread stretch or shrink when the sidewall deforms?
Although the tire won't be in a perfect circle anymore, screwing up attempts to use radius or diameter to calculate anything, the distance around the center of the tread (circumference) hasn't changed when you are on the bike - it's just in a different shape. Of course, this is very easy to verify by doing a rollout measurement.
JB