Tire width scam from all manufacturers.

how about a specification where manufacturers publish the bead-to-bead width of the tyre when pressed flat, or the arc length as inflated. then some maths whiz can publish a formula for calculating the effective diameter when accounting for inner bead width and depth.

I understand that 3T have actually proposed a new measuring system to address this problem. They call it RAM and WAM (I know, I know, and no, I'm not making this up). That is Width As Measured on Rim As Measured. 

Basically, manufacturers will be required to give the actual width of a tyre on 2 or 3 known rim widths; so a tyre might be RAM 17/WAM 24, RAM 19/WAM 26 etc. Presumably this will be at a given pressure, too (PAM?)

It's not foolproof, but it would be a pretty good starting point.

It is, in fact, an idea that makes so much logical sense it will almost certainly never happen.

This has unfortunately been going on for ages. From Sheldon Brown:

https://www.sheldonbrown.com/tire-sizing.html

Competitive pressures have often led to inaccuracy in width measurement. Here's how it works: Suppose you are in the market for a high-performance 700 x 25 tire; you might reasonably investigate catalogues and advertisements to try to find the lightest 700-25 available. If the Pepsi Tire Company and the Coke Tire Company had tires of equal quality and technology, but the Pepsi 700-25 was actually a 700-24 marked as a 25, the Pepsi tire would be lighter than the accurately-marked Coke 700-25. This would put Pepsi at a competitive advantage. In self defense, Coke would retaliate by marketing an even lighter 700-23 labeled as a 700-25.

This scenario prevailed throughout the '70's and '80's. The situation got so out-of-hand that cooler heads have prevailed, and there is a strong (but not universal) trend toward accurate width measurements.

Some road bicycles have extremely tight clearances and will not fit an honest 28mm tire.

In any case, how is 26.38 mm "just within" 28 mm ±1.5 mm?