Wikipedia claims that the figures shown on a fire hydrant sign are in SI units, though the example they depict would imply a 1mm pipe, which seems unlikely. From my youth I seem to remember that the upper figure was in feet and the lower in inches, which is somewhat more credible.

I'll check the upper distance figure, though I'm not going to open the hydrant lid up to see how big the pipe is. Anyone know what the correct unit is?