66 Barracuda Formula S - Performance indicator instead of Tach?

This reminds me of the time I left my former 66 Barracuda, slant six/auto, with my auto painter son-in-law for a fresh paint job. After he painted it, he delivered it to me, complaining, "something's wrong with the tachometer!" Huh? I says, what tachometer?

He thought the vacuum gauge was a tachometer.