No, this isn't like the sound of one hand clapping. Several popular ham antennas depend on multiple sections of coax, which act alternately as radiators and delay lines. The velocity of propagation in the delay lines is well known, being 66% for solid dielectric coax and 80 to 82 percent for foamax. But what about the sections where the signal travels on the outside of the coax in order to be radiated? How should we calculate the length of those sections? I did some quick experiments to find out. All measurements were made at 200 MHz over a dirty workbench:

Tinned copper number 22 wire was the fastest at 95%.

PVC coating of any kind slowed things down to 92%. This was equal for number 22 PVC coated and for the outside conductor of Belden 1694A coax. It compares pretty well to the widely published 93% for plastic-coated wire.

NOTE: These results were taken in the middle of a wire and to not include end effect. Including end effect and VP, a practical antenna might be 10-13% shorter than a free-space wavelength.