Originally Posted by steelbikeguy

I'm not sure exactly when it happened, but many tire manufacturers have "adjusted" their tire sizes to accommodate the greater widths of modern rims. I've had a number that ended up a couple of mm smaller than the marked size when mounted on a vintage rim.

One example is the Veloflex Corsa Evo (formerly known as the Master)... I bought some 25mm versions and mounted them on some Mavic MA-2. These ended up measuring closer to 23mm.

My point... some new 23mm tires may work to replace a 20mm tire on vintage rims, although it would be work to find out which ones would do it.