How many letters does the top answer for “measuring instrument for indicating speed of rotation” have?

The most common answer is tach (4 letters).

Crossword clue: measuring instrument for indicating speed of rotation

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

tach 4 letters wn-def

measuring instrument for indicating speed of rotation

tachometer 10 letters wn-def

measuring instrument for indicating speed of rotation