CREATE TABLE table_28787871_3 (production_code INT, us_viewers__millions_ VARCHAR)

What is the highest production code of the episodes having a US viewership of exactly 2.3?
SELECT MAX(production_code) FROM table_28787871_3 WHERE us_viewers__millions_ = "2.3"
SELECT MAX("production_code") FROM "table_28787871_3" WHERE "2.3" = "us_viewers__millions_"