Tire pressure meas (3)

I believe the answer is:

psi

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

I've seen this clue in the L.A. Times Daily, the Canadiana, the Universal, the USA Today, the Newsday.com, the LA Times and the Newsday.

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