Crossword Genius

Tire meas (3)

Ross

I believe the answer is:

psi

(Other definitions for psi that I've seen before include "Pounds per sq. in. (abbr.)" , "one employed by Socrates?" , "Greek letter; pressure unit" , "Twenty-third letter of Greek alphabet" , "letter from abroad" .)

I've seen this clue in the Newsday.com, the LA Times and the New York Times.
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