When I had my 84 Regal with the n/a 3.8 liter, the window sticker (I had it too) said 21 mpg. It did not specify if that was city or highway, but I would be willing to bet it was highway.
My car had a TH200 and a 2.41 rear end gear, 2 bbl carb.
Power wise the n/a 3.8 2bbl was rated at a tire smoking 110 hp at 3800 rpm and a frame twisting 190 ft lbs of torque at 1600 rpm.