Pilot Bob is running 5:38's and turning 33's with a 3.4 and an automatic. He is very happy with the combination.
He says it will cruise highway speeds all day and still climb like a goat with the Tera Lows. IIRC he reported the milage at about 16 mpg. 17 if the wind is low. And that's pulling a trailer to the Davis Mountains.
I even called him to confirm my info is correct.

Mileage will always get better the more vaccuum you can pull on the intake. High Manifold Vaccuum # means a happy engine.
Lug the motor and you kill your milage. Enigine is working too hard.