Fowler is talking about the English word speed being used with a Greek root meter 'measure' to create the word speedometer which is a barbarism. Like television mixes Greek 'far' and Latin 'seeing', etc. I guess tachometer had already been coined from τάχος (takhos), ταχύς (takhus) 'speed'. And Latin velocitas, velocitatis, 'speed'.

Ceci n'est pas un seing.