/6 Mileage ?

Quoted from Allpar:

"The Feather Dusters were made in 1976, with an aluminum inner hood, trunk bracing, bumper brackets, and intake manifold; economy carb and distributor calibration; large exhaust system; tall rear axle ratio; and either 3 speed auto or 4 speed overdrive manual trans.

Their gas mileage was great for their day, and even for the present, given their size: 24 city, 36 highway."

30mpg highway from a good running /6 doesn't sound unreasonable to me.