66 Barracuda Formula S - Performance indicator instead of Tach?

Hmm, I didn't think so. The dealer book says "an optional tachometer is available with V8 engines". In 67-69, at least, there were no interior trim or instrument features included in the Formula S package. It only included the suspension upgrades, badges, and a hi-po engine.

Im just posting what Plymouths own sales literature from 1966 says.

It specifically says that a Tach is standard on the 66 Formula S, here:
http://www.hamtramck-historical.com...ershipsPlymouthOrderingInfo.shtml?load_img=55

And here:
http://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1966/66_Barracuda_0008.jpg