Originally Posted by T-Mar

Stronger alloys can be made thinner to reduce weight and increase resilency. Most tubesets use single butted seat tubes, allowing one to use the seat post diameter as a gauge for determining tubing grade. The larger the post diameter, the better the grade of tubing. Knowing the post diameter, one can calculate the tubing thickness, then reference tubing manufacturer's charts to determine the tubeset.

BTW, the standard outside diameter of an imperial seat tube is 28.6mm. Your variation could be due to any number of factors, including tube distortion, abnormally thick paint, measurement technique, etc. When calculating wall thickness, use 28.6mm as your seat tube outer diameter and allow for 0.2mm clearance between the seat post and seat tube.

Tubing wall thickness = (28.6 - 0.2 - post diameter)/2

For example, a 27.2 mm seat post would result in a wall thickness of 0.6mm. Now all we would have to do if know the tubing manufacturer, then we could go to catalogues to determine which tubesets used 0.6mm wall thickness on the non-butted end of the seat tube.